Showing papers on "Leaf spot published in 1999"

Biological control of Bipolaris sorokiniana on tall fescue by Stenotrophomonas maltophilia strain C3.

Abstract: Stenotrophomonas maltophilia strain C3 was evaluated for control of leaf spot on tall fescue (Festuca arundinacea) caused by Bipolaris sorokiniana. In growth chamber experiments, C3 inhibited conidial germination on leaf surfaces and reduced lesion frequency and percent diseased leaf area compared with nontreated controls. The amount of leaf spot suppression was related to the C3 dose applied. The highest dose tested, 109 CFU/ml, prevented nearly all B. sorokiniana conidia from germinating on treated leaf surfaces and provided nearly complete suppression of lesion development. When colloidal chitin was added to C3 cell suspensions of 107 or 108 CFU/ml, biocontrol efficacy was significantly increased over C3 applied alone, whereas addition of chitin to a C3 cell suspension of 109 CFU/ml had no effect. In field experiments, application of C3 to tall fescue turf resulted in significant reductions in infection frequency and disease severity compared with nontreated controls. Strain C3 applied at 109 ...

Effects of fungicide spray timing on winter wheat disease control

Abstract: A field experiment of novel systematic design was established on 17 sites in the three years 1988–90 to compare the effect of spray timing on the disease control provided by fungicides. At each site one set of plots received applications of a mixture of fenpropimorph and prochloraz or triadimenol, tridemorph and chlorothalonil fungicides, applied at successively later start dates to produce a series of temporally and spatially related, but discrete, epidemics curtailed at consecutively later stages of development between GS32 and GS75. A second set of plots was treated with a single spray of one of a set of the nine candidate fungicides between GS32 and GS71. Disease developed at each site, with moderate septoria leaf spot (Septoria tritici) at five of the eight sites in 1988 and four of the seven sites in 1989. Erysiphe graminis f.sp. tritici was present at two sites in 1988, four sites in 1989 and one site only in 1990. Puccinia striiformis (four sites only in 1989 and 1990) and brown rust (one site in each year) were less common. The systematic treatments showed consistent relationships between green leaf retention during grain filling and yield. They also provided clear indications that epidemics of foliar disease initiated before flag leaf emergence had the greatest impact on yield. After this stage, yield loss averaged 27.2 kg ha−1 for each day that elapsed before fungicide was applied. The sequential single spray treatments showed marked differences between the protectant and eradicant activity of the different fungicides used. Each disease was most effectively controlled on the final leaf by treatment at or immediately after its emergence. Chlorothalonil was as effective as any of the fungicides tested against septoria, when applied before flag leaf emergence, but was inferior when applied after this stage. The fungicides were equally effective against yellow rust when applied just before symptoms were detected, when spray timing appeared to be more important than choice of fungicide. The response of brown rust to fungicides was similar, except that chlorothalonil was the least effective. For mildew, timing seemed to be less important, although sprays applied immediately after leaf emergence provided the best control on each individual leaf layer. Yield was well related to disease and green leaf area late in the season, these factors being more important than which disease was present, which fungicide, and which cultivar were used.

Effects of temperature and wetness duration on infection of oilseed rape leaves by ascospores of Leptosphaeria maculans (stem canker)

Abstract: In controlled environment experiments, ascospores of Leptosphaeria maculans(stem canker) infected oilseed rape (cv Nickel) leaves and caused phoma leaf spots at temperatures from 8 Ct o 24C and leaf wetness durations from 8 h to 72 h The conditions that produced the greatest numbers of leaf spot lesions were a leaf wetness duration of 48 h at 20C; numbers of lesions decreased with decreasing leaf wetness duration and increasing or decreasing temperature At 20C with 48 h of leaf wetness, it was estimated that one out of four spores infected leaves to cause a lesion whereas with 8 h of leaf wetness only one out of 300 spores caused a lesion As temperature increased from 8 Ct o 20C, the time from inoculation to the appearance of the first lesions (a measure of the incubation period) decreased from 15 to 5 days but leaf wetness duration affected the length of the incubation period only at sub-optimal temperatures Analyses suggested that, within the optimal ranges, there was little effect of temperature or wetness duration on incubation period expressed as degree-days; the time until appearance of 50% of the lesions was ca 145 degree-days A linear regression of % leaves with lesions (Pl) (square-root transformed) on % plants with lesions (Pp) accounted for 93% of the variance: p PlD 1:31C 0:061Pp This relationship was also investigated in winter oilseed rape field experiments in unsprayed plots from October to April in 1995/96 (cv Envol), 1996/97 (cv Envol), 1997/98 (cvs Bristol and Capitol) and 1998/99 (cvs Apex, Bristol and Capitol) seasons The linear regression of % leaves with lesions (square-root transformed) on % plants with lesions accounted for 90% of the variance and had a similar slope to the controlled environment relationship: p PlD 0:81C 0:051Pp These results were used to examine relationships between the development of phoma leaf spot on plants in winter oilseed rape crops, the incubation period of L maculans and the occurrence of infection criteria (temperature, rainfall) in the autumns of 1996, 1997 and 1998

Aetiology of leaf spot of garlic and onion caused by Stemphylium vesicarium in Spain

Abstract: Surveys between 1989 and 1993 in the major garlic production areas of Spain identified a new leaf spot disease, characterized by white and purple lesions followed by extensive necrosis. Isolation and pathogenicity tests with fungal isolates taken from these spots indicated that Stemphylium vesicarium was the causal agent. Pseudothecia of the teleomorph stage, Pleospora sp., were found on leaf debris from affected plants. Inoculation of garlic and onion plants with residues carrying mature pseudothecia, or with ascospore suspensions obtained from the pseudothecia, resulted in the development of white and purple leaf spots. Wetness periods longer than 24 h were required for symptom development under controlled conditions. Isolates of S. vesicarium from garlic, onion and asparagus caused disease in all three hosts. In garlic, cv. Blanco de Vallelado was most susceptible, while lines B4P17 and B6P1, and cvs Iberose and Golourose were less susceptible to the disease.

QTL analysis of Cercospora leaf spot resistance in sugar beet

Abstract: The inheritance of Cercospora leaf spot resistance in sugar beet was investigated by means of quantitative trait loci (QTL) analysis of a segregating population of 204 individuals, using 221 AFLP and 46 restriction fragment length polymorphism (RFLP) markers. Five QTL were found through composite interval mapping on linkage groups 1, 2, 3, and 9, respectively, two of which were linked on linkage group 3. The significance of these QTL was tested by permutation analysis. The QTL had mostly additive, but also certain negative dominance effects: all the resistance alleles came from the Cercospora-resistant parent. Each quantitative trait locus accounted for 7-18% of the phenotypic variation, leaving 37% of the variation unexplained. The results are discussed in relation to the potential use of marker-assisted breeding for Cercospora leaf spot resistance in sugar beet.

A Serious Outbreak of Blast Disease (Gray Leaf Spot) of Perennial Ryegrass in Golf Course Fairways in Pennsylvania

Abstract: Blast disease (gray leaf spot) of perennial ryegrass (Lolium perenne L.), caused by Pyricularia grisea (Cooke) Sacc., in golf course fairways in Pennsylvania, was first reported in 1991 (1). The disease was primarily confined to the southeastern region of the state. In August and September 1998, severe outbreaks of blast disease occurred in perennial ryegrass fairways in numerous golf courses in the same region in Pennsylvania. Additionally, in 1998, the disease was diagnosed in ryegrass fairways for the first time in the western region of the state, where serious disease outbreaks occurred. The disease develops on leaf blades as small, watersoaked lesions that become necrotic spots. The spots expand rapidly, and develop into gray, grayish-brown, or light brown, circular spots with purple to dark brown borders often surrounded by a yellow halo on the leaf blades. As the disease progresses, the circular necrotic spots coalesce, become irregular in shape, and cause partial (tip blight) or complete blighting of the leaf blades. The blighted blades may often appear twisted. Turfgrass loss due to the disease was over 90% in several golf courses. During the 1998 epidemics, a survey was conducted in the affected regions, which included three golf courses in western Pennsylvania and 15 golf courses in southeastern Pennsylvania. P. grisea was consistently isolated from the symptomatic ryegrass leaf blades of turf samples (12 to 28 samples per location) collected from the blighted fairways. Of the 122 isolates of P. grisea collected from the 18 golf courses, seven isolates (two from western Pennsylvania and five from southeastern Pennsylvania) were selected for pathogenicity assays. Five Pennfine perennial ryegrass plants were grown in each Cone-Tainer (4 mm in diameter) that was filled to 1 cm below the rim with granular calcine clay medium (Turface MVP; Allied Industrial Material, Buffalo Grove, IL). Three weeks after seeding, grasses were fertilized with water-soluble 20-20-20 fertilizer (1.3 g per liter of water) once a week to field capacity of the growing medium. Treatments (the isolates) were arranged as a randomized complete block with four replications (a Cone-Tainer per replication). Six-week-old ryegrass plants were atomized with an aqueous suspension of P. grisea conidia (approximately 8 × 104 conidia per ml of sterilized, distilled water) until the leaves were completely wet. Individual Cone-Tainers were placed in clear polyethylene bags, enclosed, and were placed in an incubator that was maintained at continuous 28°C and 12-h-day fluorescent light (72 μE s-1 m-2). Three days after inoculation, water-soaked lesions (<2 mm in diameter) developed on leaves of the ryegrass inoculated with each isolate of P. grisea. No lesions developed on leaves of the control plants. Seven days after inoculation, the polyethylene bags were removed, and disease incidence (percent infected leaves) was assessed. P. grisea was isolated from the necrotic lesions or the blighted leaf blades of every plant inoculated with the fungus. This is the first report of blast disease outbreak in golf course fairways in western Pennsylvania. The impact of blast disease epidemics on golf courses in the northeastern United States in 1998 was significant, and caused serious concern to turf managers. The survey indicates that blast disease of perennial ryegrass may be emerging as a new problem for the turfgrass industry in the northeastern United States. Reference: (1) P. J. Landschoot and B. F. Hoyland. Plant Dis. 76:1280, 1992.

First Report of Glomerella Leaf Spot (Glomerella cingulata) of Apple in the United States

Abstract: In August 1998, severe leaf spot, resulting in extensive defoliation, was observed on cv. Gala apple trees in two orchards in eastern Tennessee. Symptoms were similar to those reported in Brazil for Glomerella leaf spot on Gala (1), and Glomerella cingulata (Stoneman) Spauld. & H. Schrenk was observed fruiting in lesions. Single-ascospore isolates were obtained by placing individual perithecia from different lesions on microscope slides in a drop of sterilized distilled water. Perithecia were crushed, and the spore and mycelial suspension was distributed on the surface of petri dishes containing acid-water agar (AWA). Petri dishes were incubated at 24°C in light for 24 h. Germinated ascospores were transferred to petri dishes containing AWA. Cultures were transferred to potato dextrose agar and grown at 24°C in light for 14 days to induce sporulation. Four isolates (TN-1A, TN-1B, TN-2A, and TN-2B) were selected for pathogenicity tests. Three virulent isolates from Brazil (FK6, R-11, and 2VGE) also were included. Trees of apple cvs. Gala and Golden Delicious were placed in humidity chambers before inoculation. After 24 h, shoots on three trees of each cultivar were sprayed with an aqueous spore suspension of each isolate (1× 105 spores per ml) and maintained at 100% relative humidity (RH) and ≈22°C for 2 days. Shoots on three trees sprayed with sterilized distilled water and maintained at 100% RH served as a control. All isolates from Brazil and isolates TN-1A and TN-1B caused symptoms characteristic of Glomerella leaf spot on both cultivars after 2 days. Five days after inoculation disease severity on each leaf was visually rated on a scale of 0 to 5 (where 0 = no lesions and 5 = 25 to 50% of leaf surface covered with lesions). FK6 and R-11 were the most aggressive isolates on both cultivars. 2VGE and TN-1B were the least aggressive isolates. Reference: (1) T. B. Sutton and R.M. Sanhueza. Plant Dis. 82:267, 1998.

Identification of a novel source of resistance to angular leaf spot disease of common bean within the secondary gene pool

Abstract: Phaeoisariopsis griseola (Sacc.) Ferr., the agent of angular leaf spot disease of common bean, is a highly variable pathogen for which resistance gene diversification is required. This study analysed genetic resistance to this disease within genotypes of three Phaseolus species. Twenty-nine genotypes of Phaseolus vulgaris, Phaseolus coccineus and Phaseolus polyanthus were inoculated with 54 isolates of Phaeoisariopsis griseola, The genetic resistance was estimated according to the symptom intensity observed for each plant genotype-pathogen isolate combination. Globally, genotypes of the common bean secondary gene pool were resistant to a higher number of isolates than common bean varieties. Interactions between plant genotypes and pathogen isolates suggested vertical resistance genes within P. vulgaris, as well as within P. coccineus and P. polyanthus. The NI666' accession (P. coccineus) showed resistance to all the fungal isolates inoculated while the variety 'Aroana' (P. vulgaris) was susceptible to most of the isolates. Interspecific hybridization between these two genotypes gave F, hybrid plants which showed resistance to angular leaf spot disease.

Effects of tillage method and fallow frequency on leaf spotting diseases of spring wheat in the semiarid Canadian prairies

Abstract: Leaf spotting diseases of wheat (Triticum spp.) are widespread in western Canada. Because these diseases are residue-borne, they are expected to be affected by changes in the quality and quantity of crop residues. A field study was conducted to determine the effects of summerfallow and tillage practices on leaf spotting diseases of spring wheat (T. aestivum L.) in the semiarid area of the western Canadian prairies. Leaf spot severity was greater in wheat grown after fallow than in continuous wheat when these systems were managed using either cultivator- or zero-tillage methods. Disease severity in wheat after fallow was similar in all three tillage methods: cultivator-, reduced-, and zero-tillage. Pyrenophora tritici-repentis (Died.) Drechs. (tan spot) was the pathogen most commonly isolated from lesioned leaf tissue. Crop residues collected in the spring of 1995 and 1996 from cultivator- and zero-tillage treatments were examined for the presence and density of fungal infective structures. The density of mature and immature structures, especially of P. tritici-repentis, was greater in residues from two years previous than in those from the previous growing season. Most of the residues in the continuous wheat system were from the previous crop. The apparent lower amount of initial inoculum available in a continuous wheat system than in wheat grown after fallow would explain the higher leaf spotting severity in the latter system. In addition, lower levels of infective structures on residues were found in wheat after fallow in zero- rather than in cultivator-tillage. However, similar disease levels in cultivator- and zero-tillage treatments suggest that the more favourable microclimate for disease development in a zero-tillage system might have compensated for the lower amounts of residue-borne inoculum.

Components of rate-reducing resistance to Cercospora leaf spot in sugar beet: incubation length, infection efficiency, lesion size

Abstract: Four sugar beet cultivars exhibiting increasing levels of rate-reducing resistance towards Cercospora beticola (‘Cyrano’ or ‘Univers’, ‘Bushel’, ‘Monodoro’, and ‘Break’) were used in monocyclic infection experiments to study the following resistance components: incubation length (IP, the degree-day cumulation, base 5°C, during the time between inoculation and appearance of spots), infection efficiency of conidia (INF, the number of necrotic spots per cm2 of leaf at the end of a single infection cycle), and lesion size (LS, the area of single spots). All the resistance components were influenced by cultivar significantly; whereas no differences in lesion type were found. Resistance delayed the appearance of spots by a maximum of 12 days compared to the susceptible cultivar. IP50 was delayed by a degree-day cumulation of 28°C, 60°C and 100°C in ‘Bushel’, ‘Monodoro’, and ‘Break’, respectively. More resistant cultivars showed a reduced INF compared to both less resistant and susceptible cultivars, of about 44% in ‘Monodoro’ and 79% in ‘Break’. Furthermore, in ‘Break’ the LS was 45% smaller than that of the susceptible cultivar. Differences between cultivars were consistent from one experiment to another. Within cultivars variability was an important source of variation; it was attributed to both ‘between plants’ and ‘between leaves within plants’ variability. The former was due chiefly to the genetic heterogeneity of the cultivars used, which are open-pollened three-way cross hybrids; the latter, to differences in the age of the leaves of a plant.

Blackgram (Phaseolus mungo) germplasm evaluation against different diseases.

Abstract: One hundred diverse stocks of blackgram [Phaseolus mungo 1.] were evaluated during 1995-97 against 5 diseases widely prevalent in Himachal Pradesh. In all 13, 18, 18,25 and 31 genotypes of black gram showed varying levels of resistance to anthraenose,Colletotrichum lindemuthianum (Sace. & Magnus) Brioso & Cavara, Cercospora leaf spot, Cercospora canescens (Ell. & Martin, Mulder & Holliday) and C. cruenta (Sacc.), powdery mildew, Sphaerotheca fuliginea (Schlecht ex Fr Poll), web blight, Rhizoctonia solani Kuhn and mung yellow mosaic virus and mung yellow mosaic gemini virus respectively. Four genotypes were found to be resistant to 4 different diseases, viz 'TEU 95-1 ' (anthracnose, powdery mildew, Cercospora leaf spot and mung yellow mosaic virus), 'HPBU 51' and 'P 38' (Cercospora leafspot, powdery mildew, web blight and mung yellow mosaic virus) and 'UPU 95-1' (anthracnose, Cercospora leaf spot, powdery mildew and mung yellow mosaic virus). Nine genotypes were resistant to two diseases ie 'HPBU 38', 'HPBU 153' ,'LBG 626' and' UG 361' (mung yellow mosaic virus and web blight) 'HPBU 78' (mung yellow mosaic virus and powdery mildew), 'HPBU 98'(Cercospora leaf spot and powdery mildew), 'P 44' (anthracnose and web blight), 'WBG 57' (Cercospora leaf spot and web blight), 'WVG 108' (Cercospora leaf spot and mung yellow mosaic virus), whereas 5 genotypes were resistant to 3 diseases, viz 'KU 305' and 'WVG 110' (anthracnose, Cercospora leaf spot and mung yellow mosaic virus), 'Pusa 3' and 'UPU 91-7' (anthracnose, mung yellow mosaic virus and web blight) and 'uG 407' (Cercospora leaf spot, mung yellow mosaic virus and powdery mildew). Interestingly, genotypes 'HPBU 70', 'HPU 153', 'KU 305" 'P 38', 'Pusa 3', 'UPU 91-7' and 'WVU 110' gave higher yield/plant.

Evaluation of Yield and Resistance to Powdery Mildew, Cercospora Leaf Spot and Cowpea Weevil in Mungbean Mutant Lines

Abstract: Ten mungbean mutant lines obtained from irradiation and chemical substances were tested against the 2 recommended varieties on resistance to the diseases and insect namely powdery mildew, Cercospora leaf spot and cowpea weevil. The 2 diseases using artificial inoculation were investigated in the greenhouse while cowpea weevil was studied under laboratory conditions. Apart from these, regional yield trial were also conducted at 7 experimental stations during the dry and rainy seasons of 1998. The investigation revealed the tested mutant lines to have potential development into new varieties. They were, M5-10 and M5-25 resistant to powdery mildew, M5-22 and M5-25 resistant to Cercospora leaf spot, M5-16 and M529 resistant to cowpea weevil and the other 5 lines of M4-2, M5-1, M5-5, M5-15 and M5-28 with trend of producing good yields.

Identification and toxicity of Alternaria brassicicola, the causal agent of dark leaf spot disease of Brassica species grown in Thailand

Abstract: Fungi, with spores characteristic of the genus Alternaria, were isolated from necrotic lesions on leaves of cabbage, cauliflower, Chinese kale and choi-sum growing in Thailand, and were proved by Koch’s postulates to be the causal agents of a disease known as dark leaf spot All isolates corresponded in morphology to descriptions of Alternaria brassicicola and the identification was confirmed by analysis of the ITS1, 5·8S gene and ITS2 regions of rDNA, the nucleotide sequences of isolates from all four plants being identical to each other and to the published sequence of a known isolate of A brassicicola Culture filtrates of isolates of the fungus from each host, grown on a defined medium consisting of Czapek‐Dox nutrients supplemented with cations, were toxic to cells isolated from the four host plants Taking the data overall, filtrates from the cauliflower isolate were significantly less toxic than those from the other isolates Although the filtrate from the cabbage isolate was most toxic to cabbage cells and that of the choi-sum isolate most toxic to choi-sum cells, filtrates of the Chinese kale and cauliflower isolates were most toxic to cells of plants other than those from which they were isolated

Vulnerability of U.S. Maize Germ Plasm to Phaeosphaeria Leaf Spot

Abstract: Carson, M. L. 1999. Vulnerability of U.S. maize germ plasm to Phaeosphaeria leaf spot. Plant Dis. 83:462-464. Phaeosphaeria leaf spot (PLS) is a potentially important maize disease that has recently appeared in the continental United States in winter breeding nurseries in southern Florida. To better predict the potential of this newly introduced disease to inflict damage on the U.S. maize crop, 64 public and private inbred lines and 80 proprietary commercial maize hybrids representing the genetic diversity in the U.S. maize crop were evaluated for resistance to PLS in the 1996-97 and 1997-98 winter nursery seasons. Plots were evaluated for PLS severity (0 to 9 scale) at the early to mid dent stages of kernel development. Relatively few hybrids or inbreds were free from PLS at this growth stage. Inbred lines related to B73 were particularly susceptible to PLS. Relatively few commercial hybrids were as severely diseased as a susceptible check hybrid, indicating that U.S. maize production is not particularly vulnerable to damage from PLS at this time. However, the susceptibility of several widely used parental inbred lines makes PLS a potential concern to the seed industry should it become established in areas of hybrid seed production.

Registration of FC709-2 and FC727 Sugarbeet Germplasms Resistant to Rhizoctonia Root Rot and Cercospora Leaf Spot

Abstract: Sugarbeet (Beta vulgaris L.) germplasms FC709-2 (Reg. no. GP200, PI 599668) and FC727 (Reg. no. GP-201, PI 599669) were developed by the USDA-ARS, Fort Collins, CO, in cooperation with the Beet Sugar Development Foundation, Denver, CO. These lines should provide excellent resistance to root-rotting strains (AG2-2) of Rhizoctonia solani Kiihn and moderate resistance to cercospora leaf spot (caused by Cercospora beticola Sacc.). They are potential pollinators or populations from which to select pollinators with combining ability for yield. FC709-2 and FC727 were released in 1998 from seed productions 9210124 and 951017, respectively. FC709-2 is multigerm (MM), non-O-type, and pseudo-selffertile, with 13% green hypocotyls. It is segregating, with approximately 19% male sterility. The sterility is most likely cytoplasmic, because plants had a range of anther types from white, shrunken anthers with no pollen production to full, yellow, dehiscent anthers with normal pollen. However, no progeny testing was done to confirm the basis of sterility. FC709-2 is the result of three cycles of selection within the line 871016. 871016 had one more cycle of selection for resistance to R. solani than its parent line, 841034. FC709 (1) was released as a composite of seed from 871016 and 841034. The first cycle of selection for FC709-2 was mass selection for resistance to rhizoctonia root rot in a nursery inoculated with R. solani (2). Seed was produced in bulk from surviving roots planted in a field isolation plot where they were reinoculated with R. solani. Seed from 28 plants was harvested. Some of this seed was planted into a disease-free nursery and, based on percent sucrose of individual roots, the 9.4% of beets with the highest sucrose (55 beets) were selected. The remaining seed from Cycle 1 was grown in a rhizoctonia-inoculated nursery, and three beets were selected. The three beets were increased in bulk along with the 55 beets selected for high sucrose. Seed was harvested from a total of 33 surviving plants. This Cycle 2 population underwent another cycle of mass selection for resistance to rhizoctonia root rot in an inoculated nursery (2). Seed from the 121 surviving plants was harvested in bulk to produce FC709-2. FC709-2 had high resistance to rhizoctonia root rot when tested under strong disease pressure (2). Based on disease index (DI) ratings from 1994 through 1997, where a DI of 0 = no root rot and 7 = all plants dead, there were either no significant differences or FC709-2 performed better than the rhizoctonia-resistant controls. In these four years, the DI ratings for the resistant check (FC705-1) were 1.4,1.4,1.5, and 3.2, compared with 1.0,1.5, 0.9, and 2.5 for FC709-2. FC709-2 was always significantly better than the susceptible control (FC901/C817//413). Of all lines tested, FC 709-2 had the lowest DI rating in the very severe epiphytotic of 1997. Percentages of resistant plants (those rated 0 or 1) were 86, 55, 100, and 49 for FC709-2, and 65, 58, 62, and 49 for FC705-1. FC709-2 showed good resistance to cercospora leaf spot when tested in artificial epiphytotics (3). In three years of testing, it was significantly better than the susceptible control and not significantly different from the resistant control. The following DI ratings (DI of 0 = no leaf spot and 10 = all plants dead) are from the most severe rating (the last of three or four ratings each season). In 1994, 1995, and 1997, DI ratings of FC709-2 were 3.0, 4.3, and 4.2/3.5 (tested in two trials). Ratings of the resistant control (FC504CMS/FC502-2//SP6322-0) were3.2,3.5, and3.8/2.9 forthe same respective years, and ratings of the susceptible control (SP351069-0) were 4.5, 6.2, and 7.0/6.5. FC709-2 does not have tolerance to the beet curly top virus (BCTV). As with FC709, field testing of FC709-2 in California revealed a low frequency of plants with some resistance to rhizomania (R.T. Lewellen, personal communication); however, this germplasm is not recommended as a source for rhizomania resistance. In a 1995 yield trial at Fort Collins, CO, with moderate drought stress, the sucrose yield of FC709-2 was 89% of 'Beta 2398' and 66% of 'Monohikari' and the percent sucrose was 102% of 'Beta 2398' and 86% of'Monohikari\". FC709-2 has not been tested for combining ability. It was released for use as a pollinator for making rhizoctonia root rotand cercospora leaf spot-resistant hybrids, or as a source population from which such pollinators can be selected. FC727 is multigerm (MM), non-O-type, and has 51% green hypocotyls. FC727 resulted from crossing FC703 (4) (50% of the genetic contribution) to three high-sucrose sources: Polish AJ, ZZ (16%), the Spanish line 'Aula Dei 13' (21%), and 'American Crystal 67-436' (13%). FC727 is the result of eight cycles of mass selection for resistance to rhizoctonia root rot. Within four of these cycles, resistant roots were simultaneously selected for high sucrose of individual roots. The smallest population size was 26 plants and, on average, a selection intensity of 18% for sucrose was used. FC727 has excellent resistance to rhizoctonia root rot when tested under severe disease pressure (2). There were no significant differences between it and rhizoctonia-resistant controls in DI ratings, and FC727 was significantly more resistant than the susceptible control (FC901/C817//413) from 1994 through 1997. FC727 had mean yearly DIs of 1.4, 1.7, 1.1, and 3.6 (19941997), whereas the highly resistant control (FC705-1) had DIs of 1.4,1.4, 1.5, and 3.2. Percentages of resistant plants (those rated 0 or 1) were 69, 41, 89, and 16 for FC727 and 65, 58, 62, and 49 for FC705-1. FC727 has a moderate reaction to cercospora leaf spot. In three years of testing in an artificial epiphytotic of cercospora leaf spot (3), it was significantly more resistant than the susceptible control but, significantly more susceptible than the resistant control. The following DI ratings are from the most severe rating (last of three or four ratings each season). In 1994, 1995, and 1997, DIs of FC727 were 3.8, 4.5, and 4.8. DIs of the resistant control (FC504CMS/FC502-2//SP6322-0) were 3.2, 3.5, and 2.9. DIs of the susceptible control (SP351069-0) were 4.5,6.2, and 6.5. FC727 does not have tolerance to BCTV. FC727 had relatively good general combining ability for sucrose yield when used as a pollinator on several diverse CMS lines. FC727 has potential for use as a pollinator or a population from which to choose pollinators with good combining ability. It should contribute to the development of high-sucrose hybrids with resistance to rhizoctonia root rot. Seed of FC709-2 and FC727 is maintained by the USDA-ARS and, for at least five years, will be provided in quantities sufficient for reproduction upon written request to Sugarbeet Research, USDA-ARS, Crops Research Laboratory, 1701 Center Ave., Fort Collins, CO 80526-2083. Seed of this release will be deposited in the National Plant Germplasm System, where it will be available for research purposes, including development and commercialization of new lines or cultivars. The developing organizations request appropriate recognition of the source when this germplasm contributes to a new cultivar.

Ascospores as primary inoculum for epidemics of white leaf spot (Mycosphaerella capsellae) in winter oilseed rape in the UK

Abstract: In the UK, conidia of Pseudocercosporella capsellae, the anamorph of Mycosphaerella capsellae, were observed on white leaf spot lesions on leaves throughout the growing season. Ascomata were not observed on lesions on either green or senescent leaves, although stromatic knots and spermogonia were occasionally seen in summer. However, spermogonia and protoascomata were produced in white leaf spot pod and stem lesions in early summer. Protoascomata continued to mature after harvest in these lesions on the debris. Mature ascomata subsequently developed by early autumn, but were exhausted by early January and did not overwinter. A diurnal periodicity in numbers of air-borne M. capsellae ascospores discharged from infected debris was observed with a Burkard spore sampler, with greatest numbers of ascospores collected near the middle of the day; the records also suggested that ascospores were released in response to wetting by dew or rain. Studies of natural white leaf spot epidemics in winter oilseed rape provided evidence that air-borne ascospores are the primary inoculum for initiating epidemics in the autumn in the UK. White leaf spot disease gradients over 100 m across a winter oilseed rape crop at Rothamsted were fitted by both negative exponential and inverse power-law models, with gradient slopes suggesting the deposition of air-borne spores dispersed from a single local source of inoculum. In comparison, no obvious white leaf spot gradients were observed over 250 m in a severely diseased crop near North Petherton, Somerset, suggesting that the air-borne spores were dispersed from a number of more distant sources in the area. Both patterns of disease were unlikely to have been initiated by P. capsellae conidia, which are dispersed only very short distances by rain-splash. However, once epidemics have been initiated by air-borne ascospores in the autumn, subsequent disease spread within an infected crop is dependent only on splash-dispersed conidia. A revised disease cycle of the pathogen is proposed.

Septoria leaf spot of stevia rebaudiana in Canada and methods for screening for resistance

Abstract: The herb Stevia rebaudiana is a potential source of low-calorie sweeteners. In 1995, a severe leaf spot and blight was observed in stevia production fields and research plots in the provinces of Ontario and British Columbia, Canada. The disease was characterized by angular, shiny, olive-grey lesions that rapidly coalesced and were often surrounded by a chlorotic halo. Leaves quickly became necrotic and often dropped off the plant. The disease progressed upwards in the foliage during the growing season. A Septoria sp. was isolated from diseased leaves. Ten isolates (five from each of the two provinces) of the Septoria sp. were compared with respect to conidial size. Across isolates, conidia lengths and widths overlapped (grand means for length and width were 71.4 μm and 1.4 μm, respectively). Conidiogenesis was holoblastic. Morphological characteristics and disease symptoms were similar to those of Septoria steviae, previously reported only from Japan. It was concluded that the Canadian isolates belonged to S. steviae. Isolates from Canada did not differ significantly from one another with respect to effects of temperature on colony growth or germination of conidia. Optimum temperatures for these parameters were between 20 and 25°C. In field trials, the pathogen was shown to successfully over-winter in diseased leaf tissue. In order to develop procedures for identification of resistant germplasm and greenhouse screening of candidate fungicides, effects of leaf wetness period, inoculum concentration, and plant age on disease development were determined. Thirty-six hours of leaf wetness were required for consistent development of leaf spots. Inoculum concentrations of 5 × 105 conidia/ml or more were required to produce high disease severities; 6-week-old plants were more susceptible than older plants. In the growth chamber, greenhouse, and field trials, germplasm selections with high levels of resistance to S. steviae were identified. This is the first report of resistance to this disease in S. rebaudiana.

Corynespora cassiicola leaf spot of pawpaw (Carica papaya L.) in Nigeria.

Abstract: Leaf spot of pawpaw is hereby reported for the first time in Nigeria. The symptom is characterized by a papery center surrounded by a yellow halo. The causal organism is Corynespora cassiicola. Ripe fruits and abaxial surfaces of the leaves were significantly more susceptible to infection than unripe fruits and adaxial surfaces of leaves. Growth and sporulation of the fungus on several media was investigated. The organism grew faster on malt-extract agar (MEA) derived media and slowest on potato-dextrose agar (PDA) supplemented with thiamine. Sporulation was highest on Czapek-dox agar (CDA) plus biotin and lowest on PDA and PDA + thiamine. Reasons for increased susceptibility of ripe fruit are discussed.

First Report of Alternaria alternata Causing Leaf Spot on English Walnut.

Abstract: A foliar disease of English walnut (Juglans regia L.) was observed in 1998 in an orchard in northern Italy. Symptoms consisted of circular, necrotic spots bordered with concentric zones of darker tissue. Average lesion diameter was 20 mm, although lesions could extend to half of the leaflet lamina. A fungus identified as Alternaria alternata (Fr.:Fr.) Keissl. (1) was observed on the necrotic tissue and was consistently isolated from the margins of the necrosis. Conidia from leaves were brown, ellipsoid to ovoid (primary conidia 28 to 60 × 8 to 15 μm; secondary conidia 10 to 30 × 7 to 13 μm), with walls often ornamented, 1 to 6 transversely septate and 0 to 3 longitudinally septate. For pathogenicity tests, three isolates were grown on potato dextrose agar for 2 weeks. Inoculations were performed on detached, surface-sterilized, healthy J. regia leaflets. Four drops (5 μl each) of a sterile water suspension of 1 × 105 conidia per ml were placed on each leaflet; three leaves per isolate were used. Leaves were incubated in a moist chamber. After 10 days, leaf spots similar to the original symptoms developed on all the inoculated points for all three isolates, and the pathogen was reisolated. Control leaflets inoculated with sterile, distilled water remained symptomless. The experiment was performed three times and the results were similar. Alternaria alternata is a well-known pathogen on many crops but a few records report this fungus as a causal agent of leaf spot on deciduous trees. This is the first report of A. alternata on English walnut. Reference: (1) E. G. Simmons. Mycotaxon 37:79, 1990.

Efficacy of four triazole fungicides in the control of leaf spots and rust of groundnut

Abstract: Early leaf spot (ELS) caused by Cercospora arachidicola Hori. late leaf spot (LLS) caused by Phaeoisariopsis personata (Berk. & Curt.) V. Arx. and rust (Puccinia arachidis Speg.) are the most serious diseases of groundnut in Saurashtra region of Gujarat. In many areas, rust, early and late leaf spot can together caused an yield loss up to 70% (3). The present investigation was undertaken for the fungicidal control of leaf spots (ELS & LLS) and rust during 1995-1997 kharif seasons at College Farm, Gujarat Agricultural University, Junagadh on groundnut cultivar GG-2. The trial was laid out in randomized block design. The plot size was 5.0 x 3.6 m with 45 ems space between rows. The experiment was carried out with eight treatments including four triazoles viz., difenconazole (Score 25 EC) @ 0.0125 per cent a.i., propiconazole (Tilt 25 EC) @ 0.0125 per cent a.i., hexaconazole (Contaf 5 EC) @ 0.0025 per cent a.i., epoxiconazole (Opus 12.5 EC) @ 0.00625 per cent a.i., each @ 0.5 mill and two conventional fungicides viz., carbendazim (Bavistin 50 WP) @ 0.025 per cent a.i. (0.5 g/l) and mancozeb (Indofil M - 45 75 WP) @ 0.20 per cent a.i. (2.66 g/l) with two controls i.e., only water spray and unsprayed. The treatments were applied as foliar spray with high volume sprayer thrice at the interval of 15 days starting at 30 days after emergence. Observations on leaf spot and rust intensity were recorded 15 days after third spray by selecting ten plants randomly from each plot and plants were rated on 1-9 scale (6). Per cent disease intensity (PDI) was worked out using following formula: