BP Singh

Symptoms Caused by Plant Vinlses 163which expresses symptom only when the temperature is around16°C.3. Deviation~ resembling symptoms of virus disea&e: SymptolnSof certain virus diseases closely resemble deviations due to geneticaberrations, physiological non-parasitic disturbances, insecttoxemias, micro-organisms such as mycoplasma-like-organisms(MLOs), spirdplasma, rickettsia-like-organisms (RLOs), rickettsialike-bacteria(RLB) and viroids. Many of the transmissible diseasesymptoms thought to be caused by viruses were infact later provedto be caused by MLO, RLO, RLB and viroids. Sometimes. differentagents lead to identical deviations, especially with malformationssuch as enations and tumours may result from attacks by bacteria,mite, insects, nematodes or a virus. Symptoms produced by soilnutritional disorders such as mineral deficiencies and toxicities areoften confused with virus or virus-like symptoms.Despite problems encountered in identifying symptoms ofviral diseases, it should be noted that vIrus symptoms often havea characteristic distribution or the sequence of symptoms in virusinfectedplants because of the typical way in which the virusinfects, and associates with specific tissue, spreads and multipliesin the plant.Laboratory detection of viral symptoms in germplasmA variety of viral symptoms on seed such a~ discolouration,deformation, necrosis etc. on seed coat have been identified to beassociated with particular virus infection, e.g., mottling on soybeanseeds due to soybean mosaic virus (SMV) , shrivelled seeds ofmungbean and urdbean due to urdbean leaf crinkle virus (ULCV),necrotic line pattern or I and split seed coat of pea due to pea seed. borne mosaic virus (PSbMV), necrotic spots of broad bean due tobroadbean stain virus (BBSV), colouration of part of seed coat ofcowpea due to cowpea aphid-borne mosaic virus (CApMV), andpigmentation of seed coat of soybean cv. iwate-wasekurome dueto soybean stunt virus.Similarly, germplasm cuttings or tissue culture material mayalso show paleness, yellowing or virus-like symptom, but theseneed actual virus-indexing procedures to detect any association ofpathogen. Such sYlflptoms, infact, can be also caused byenvironmental factors or nutritional disorders during transit.