Showing papers on "Plant disease resistance published in 1976"

The involvement of flavour volatiles in the resistance to downy mildew of wild and cultivated forms of brassica oleracea

Abstract: SUMMARY The possible involvement of flavour volatiles in the resistance of Brassica oleracea L. seedlings to cabbage downy mildew (Peronospora parasitica (Pers. ex Fr.) Fr.) was investigated using seedlings of a number of commercial Brassica oleracea cultivars. Flavour volatiles are primarily released as a result of tissue damage in brassicas. Only one cultivar tested was resistant to the disease and its tissue macerates yielded the greatest concentration of volatiles. One of these compounds, allyl isothiocyanate, proved to be highly toxic to the pathogen and was produced in much greater concentrations in macerates of cotyledon than first leaf tissue. The ability of seedlings to produce allyl isothiocyanate in large amounts following tissue damage may inhibit the growth of Peronospora parasitica in resistant seedlings. Wild populations of Brassva oleracea L. subsp. oleracea (wild cabbage) were also tested for disease resistance, and the highest proportions of resistant seedlings were found in those populations containing high levels of flavour volatiles. The longest established wild populations possessed the highest average concentrations of flavour volatiles, and it is suggested that selective breeding has resulted in a reduction in the amounts of these compounds in cultivated forms. This change may be a contributory factor towards the lack of resistance shown by modern cultivars.

Host-specific Toxins in Relation to Pathogenesis and Disease Resistance

Abstract: Many pathogenic microorganisms have highly specific host ranges; only certain species, cultivars, or genotypes of plants are affected. In general, the chemical or physical bases of such host-specificity are unknown. In some cases, however, specificity or selectivity is related to production and release by the pathogen of toxic substances with remarkable characteristics. These are the so-called “hostspecific toxins” (Pringle and Scheffer, 1964), which damage or destroy plant tissues that are susceptible to the toxin-producing microorganism, but have little or no effect on other plants, microorganisms, or animals.

Early determination of genotypes for apple scab resistance by forced flowering of test cross progenies

Abstract: Apple selections with different major genes for resistance to apple scab (Venturia inaequalis) derived from Malus floribunda and M. pumila were crossed with each other. The progenies were screened as young seedlings for their reaction to V. inaequalis race 1. A gene for resistance from M. pumila, causing stellate necrotic (SN) lesions, was epistatic to a second gene for resistance from M. floribunda, causing irregular chlorotic (Chl) lesions. Although in most cases SN, Chl and susceptible phenotypes were clearly distinct, occasionally reactions were difficult to characterize or varied from one inoculation to another. Selected seedlings showing resistant or susceptible reactions were forced to flower in 16–20 months in the greenhouse and test crossed with susceptible cultivars. Test cross seedlings were screened for scab reaction. The presence of both genes for resistance in a resistant plant was indicated by presence of both Chl and SN resistant phenotypes in the test cross progeny. Chi-square analysis of four large progenies produced a good fit to the expected ratio. The use of the forced flowering technique to determine scab resistance genotypes in 28 months demonstrated its value in breeding apples with multiple disease resistance.

Studies on bacterial WILT I. inheritance of resistance to Pseudomonas solanacearum in tomato

Abstract: Variance component analysis of the PI,P2,F1 ,F2,B1,B2 generations of a cross between resis- tant (V-4) and susceptible (Walter)tomato cultivars indicated a narrow-sense heritability of wilt resistance of 42% and a broad-sense heritability of 53%, with a degree of dominance of 75%. A diallel cross was performed among six tomato cultivars, Walter, CRA-66, H-7741, Venus, VC-4 and Llanos de Colee, representing a range of susceptibility/resistance of 99.5 to 20.8 on a disease resistance scale. The results indicated that general combining ability (GCA) was considerably more important than specific combining ability (SCA), suggesting that inheri- tance of wilt resistance was due mainly to additive gene action. Among the six cultivars, Llanos de Colee, VC-4 and Venus showedgood GCA. Wilt resistance could attain a high level in a breeding procedure of repeated selfing and sel- ection, followed by intercrossing of resistant selections.

Analysis of Nuclear Genetic Resistance in Maize to Race T of Southern Corn Leaf Blight

Abstract: Two maize (Zea mays L.) inbred lines, in Texas malesterile cytoplasm (Tcms), with high levels of nuclear genetic resistance to race T southern corn leaf blight (Helminthosporium maydis Nisikado and Miyake) were derived by selfing and selection from two open-pollinatedcomposite sources. The lines were confirmed to be in Tcms and were utilized in crosses with an extremely susceptible Tcms line in two generation-mean analysis experiments. Resistance was polygenic, and additive genetic effects accounted for most of the variation among generations. Dominance effects accounted for a small but significant portion of the genetic variance in both experiments. The results suggest that gene frequencies for resistance could be enhanced by recurrent mass selection. In a program for development of commercial hybrids, selection should be practiced in male-sterile cytoplasm in two unrelated source populations. Lines would be extracted by inbreeding and crossed. Resulting hybrids would be evaluated for disease resistance, agronomic suitability, and yield.