PhD

The investigation on " Genotypic and seasonal influence on leafspot
disease in amaranth" was undertaken in the Department of
Olericulture, College of Horticulture, Vellanikkara, Thrissur during
1996 - 1998. The objectives of the study were to identify amaranth
accessions resistant to leaf spot disease, to isolate and identify the
pathogens associated with the disease, to study the seasonal
influence on leaf spot and yield and to find the possibility for
biochemical cataloguing of amaranth accessions.
Evaluation of 168 amaranth accessions for leaf spot disease



resulted in identification of


14 immune, 15 resistant, 19



moderately resistant, 34 moderately susceptible and 86 highly
susceptible accessions. Red types were found highly susceptible as
compared to green types. Disease infection occurred within 15 days
of planting resulted in maximum percentage of disease severity.
Two types of leaf spot symptoms were inflicted and causal
organisms identified were Rhizoctonia solani Kuhn. and
Colletotrichum capsici (Syd.) Butler and Bisby. The two organisms
either alone or in combination caused the incidence and
development of leaf spot disease. Red types were infected by
Rhizoctonia solani alone where as the green types were infected by
both pathogens.

Seasonal influence on disease recorded maximum disease severity
in July crop and minimum in April crop. Low temperature, high
relative humidity and high rainfall were the favourable weather
conditions for disease development. Leaf spot in amaranth was
negatively correlated with maximum and minimum temperatures
where as relative humidity and rainfall were positively correlated
with the disease.
Highest yield was realised in March planted crop and lowest in June
crop. Yield and yield attributes except plant height were positively
correlated with maximum and minimum temperatures where as
relative humidity and rainfall were positively correlated with them.
High content of total phenols, OD phenol and ascorbic acid were
recorded in immune and resistant accessions than susceptible.
Total phenol content was higher in rainy season. OD phenol,
ascorbic acid and pigments decreased upon infection in rainy
season. Oxalates and nitrates had no influence on leaf spot disease
development. High negative correlation was found between total
phenol, OD phenol, .ascorbic acid, chlorophyll 'a' and 'b' and
disease.
Generally immune and resistant accessions had more number of
PPO and PRX bands than susceptible types. The additional band
expressed in diseased condition in certain accessions can be taken
as the biological marker for leaf spot resistance.