Showing papers on "Plant disease resistance published in 1971"

Current Status of the Gene-For-Gene Concept

Abstract: One of the most successful means of controlling plant diseases has been the development of varieties with major or vertical resistance genes. This type of resistance is easily manipulated in a breeding program and is efIec­ tive until strains of the pathogen to which it does not confer resistance be­ come established. Then, if another gene that conditions resistance to the new strains of the pathogen is available, this resistance gene may be incorporated into the variety by the plant breeder. In doing this, the breeder either con­ sciously or unconsciously is applying the principle of the gene-far-gene hypothesis. Plants resistant to races that are virulent on old varieties possess the new resistance gene. With the diseases of some crops, this process has becn repeated at relatively frequent intervals (4D, 42, 82). However, in some instances a single gene has conferred adequate resistance for many years 80,82). In plant diseases caused by living organisms, the same phenomena: in­ fection type in rusts, percent of infected plants in smuts of cereals, fleck or lesion in apple scab, are criteria of both the reaction of the host and the pathogenicity of the parasite. They indicate the relative resistance or sus­ ceptibility of the host and the relative avirulence or virulence of the para­ site. The gene-for-gene hypothesis was proposed (20,25) as the simplest ex­ planation of the results of studies on the inheritance of pathogenicity in the .flax rust fungus, M elampsora lini. On varieties of flax, Linum usitatissimum that have one gene for resistance to the avirulent parent race, F 2 cultures of the fungus segregate into monofactorial ratios. On varieties having 2, 3, or 4 genes for resistance to the avirulent parent race, the F2 cultures segregate into bi-, trio, or tetra factorial ratios (20-22) respectively. This suggests that for each gene that conditions reaction in the host there is a correspond­ ing gene in the parasite that conditions pathogenicity. Each gene in either member of a host-parasite system may be identified only by its counterpart in the other member of the system.

Genetics of Disease Resistance in Plants

Abstract: Most plant diseases are infectious. They result from the interaction of two organisms, the host plant which is the suscept of the disease and the patho­ gen which is the causal agent of the disease. A wide array of pathogens cause disease. These include many fungi, bacteria, viruses, mycoplasma, nematodes, and parasitic phanerogams. Several abiotic environmental factors can also cause plant disease. Disease resistance may be partial or nearly complete. Complete susceptibility of a plant to a pathogen is rare. In such hosts, nothing impedes pathogen development throughout the disease cycle. A large proportion of pathogen propagules reach the host and infect it. Pathogen development is rapid and abundant in the host. The pathogen reproduces rapidly and over a prolonged period. If the pathogen has an efficient means of dispersal, it spreads rapidly to new infection courts. Under favorable en­ vironmental conditions an explosive epiphytotic results. In the resistant plant, host and pathogen are in conflict; pathogen growth and development are suboptimal. The resistance can take various forms and numerous genetic systems are known to result in host resistance. Genetic variation within the pathogen is of extreme importance in disease resistance studies and in disease control by means of host resistance (1-4). Some forms of host resistance function against certain races or biotypes of a pathogen but not all against others. In contrast, other forms of resistance function against all biotypes of a pathogen.

Genetical Approach to the Biochemical Nature of Plant Disease Resistance

Abstract: i\lost of the genetical studies on true resistance have dealt with specific r esistance that a variety with r esistance geuc (s) is resistant only to some (not all) fung us strains of a pathogen . F lor'> studied t he inheri tance of hostpathogen relationship in the flax and the flax rust system, and demonstrated the genefor-gene relationship. Such a relationship was shown in other crop diseases: powdery mildew of barley> and wheat,••> bunt of wheat,2°> stem rust of wheat, ' blast of rice,8>··"> late blight of potato/> etc. The author >· 11 > gave the fo llowing genetical

Fusarium wilt of Chrysanthemum: symptomatology and cultivar reactions.

Abstract: A greenhouse technique was developed for ob taining infection of chrysanthemum by Fusarium oxysporum f. sp. chrysanthemi and F. oxysporum f. sp. tracheiphilum race 1. Initial foliage symptoms occurred seven days after inoculation of a very susceptible cultivar and in 35 days on a more resistant one. Early symptoms included unilateral chlorosis of one or more leaves at or near the stem apex, and slight to pronounced curvature of the chlorotic leaves and the stem toward the affected side of the plant. As disease progressed, chlorosis of the affected leaves became more general and severely affected leaves wilted. Wilted leaves oc curred initially on the most severely affected side of the plant but as disease progressed the entire plant wilted and died. Black necrosis of the stem developed. It sometimes occurred as a streak up one side of the stem or as a black necrotic area in the upper part of the stem. Vascular discoloration always developed in the stems; it also occurred in the leaves of some cultivars. Cultivars with severe foliage and stem symptoms were Escapade, Nob Hill, Southern Comfort, Torch, White Marble, Yellow Bonnie Jean and Yellow Delaware. Culti vars with mild symptoms were Bluechip, Giant No. 4 Indianapolis Yellow, Iceberg, and Puritan. Culti vars with no symptoms were Dillon Beauregard, Stingray and Tinsel.

Control of Zebra Disease of Agave Hybrids by Breeding for Resistance to Phytophthora spp.

Abstract: A zebra disease resistance factor has been incorporated into the high-yielding Agave hybrid no. 11648 by crossing this hybrid with A. lespinassei. The progeny of three crosses have been screened for resistance to Phytophthora nicotianae, the chief pathogen of zebra disease and results show that 56.7% of the plants have sufficient resistance to be of economic value. The two screening techniques used are described, and the nature of the resistance factor discussed.

Wheat Breeding for Disease Resistance by Interspecific and Intergeneric Hybridization

Abstract: In various countries, efforts were made from olden days to introduce disease-resistant genes from close relat{ves of different genera into cultivating wheat varieties and in recent years excellent resu lts of breeding have been obtained especially by chromosome engineering. In wheat breeding in Japan, leaf-rust resistance is an important breeding object and a plan of wheat breeding by interspecific and intergeneric hybridization has been under way at the Tohoku National Agricultural Experiment Station since 1951 to cope with various races of wheat leaf-rust (Puccinia r econdita. Rob. ex Desm.). The progress of this work is outlined in the following :