BP Singh

136 P.c. Agarwalmay further result in stunting of the whole plant. Some pathogensproduce a wide range of symptoms. Xanthomonas campestris pv.·malvacearum causes angular leaf spot, veinal necrosis, boll rot, andstem dieback (black arm). The type and severity of symptomsproduced depends on sevet'i'l factors including climatic conditions,resistance of the host, site and method of infection and possiblythe amount of in'oculum.The mechanism involved in symptom expression is complex.Toxins may be produced to cause necrosis or chlorosis, as withFusarium oxysporum, Pseudomonas syringae pv. phaseolicola; enzymesmay be involved, as with soft rotting Erwinia spp. that producepectinases; plasmids may be involved, as with Agrobacteriumtumefaciens to produce a tumor, etc.Symptoms on seedFungal pathogens produce various types of symptoms onseed which may be specific or non-specific. Purple discolourationof a part of whole soybean seed indicates infection of Cercosporakikuchii. In case of downy mildew of soybean (Peronosporamanshurica) dull white crusts are present on the seed. Distinct 'eyespot' with grey centre and brown to dark brown margins on wheatseed shows infection of Drechslera sorokiniana (seedling blight/rootrot). Shrivelled, whitish/pinkish light weight seeds of wheat andmaize indicate infection by Fusarium spp. (foot rot, ear scab). Blackcoloured, dotted structures on the surface of groundnut, sesame,maize seed reveal infection of charcoal rot pathogen (Macrophominaphaseolina). In Karnal bunt of wheat (Neovossia indica), seed is. converted into dark brown/black powdery mass to varyingdegrees along the furrow. In case of paddy bunt (Neovossia horrida),minute pustules or streaks of black colour brush through theglumes, the seed may be partly or wholly converted into blacksp·ore mass. In the case of coriander gall disease (Protomycesmacrosporus) seed shows hypertrophy of a part of entire seed. Inthe case of wheat bunts, covered smut of barley and oat, grain smutof sorghum, pearlmillet etc. entire seed except seed coat isconverted into black powdery mass of spores, while in ergotdiseases of cereals and grasses individual grains are converted intolong, black hard sclerotia.Xanthomonas campestris pv. phaseoli in bean is detectable bythe presence of the va~nished masses of bacteria in the hilum area.