Variability in yellow leaf symptom expression caused by the Sugarcane yellow leaf virus and its seasonal influence in sugarcane

Yellowing of leaf midrib, its extending to adjoining lamina and necrosis of discoloured tissues from the tip portion are the common yellow leaf (YL) symptoms observed in tropical climatic conditions in India. As symptoms of this disease vary among the varieties, the progress of these symptoms was studied in Coimbatore from disease free status in the plant, using a newly devised YL 0-5 disease severity scale. Progress of recovery of symptoms in symptomatic leaves was monitored with a 0-4 symptom severity scale for four seasons. Detailed observations with ten symptomatic varieties have very clearly indicated a clear variation in symptom expression among them. First symptoms were recorded in leaf 1 and 2 in few plants of the cvs CoPant 84211 and B38192. Apart from yellow discoloration of midrib, reddish discoloration, pinkish midrib and laminar discoloration were recorded in cvs B38192, Co 86010 and Co 85019, respectively. Progress of laminar discoloration on both sides parallel to the midrib varied from similar to 0.5 to 3.0 cm in different varieties. Apart from severe laminar discoloration progressive leaf drying from the tip region along the midrib towards leaf base and bunching of leaves in the crown were identified as the severe form of disease in sugarcane varieties. For such leaf drying with characteristic pattern of 'V' or 'Y' shape and in a bunchy top, leaf yellowing was recorded up to -7(th) leaf and drying was recorded up to -1(st) leaf in severe YL-affected variety B38192. Progressive increase in disease symptom was noticed in most of the varieties till nine months after planting and later a fluctuation in disease expression was noticed among the varieties. Subsequently decrease of yellowing was observed leaving behind the dried laminar region at the tip. The correlation and regression analyses with meteorological parameters established that similar to 50 % of variability in disease severity grades is explained by minimum temperature and relative humidity in the afternoon. This study on the disease symptomology has clearly revealed variation in YL symptom expressions among the varieties and impact of prevailing weather factors on the fluctuation of disease expressions.