PHYTOPHTHORA-CRYPTOGEA ROOT-ROT OF TOMATO IN ROCKWOOL NUTRIENT CULTURE .3. EFFECT OF ROOT-ZONE TEMPERATURE ON GROWTH AND YIELD OF WINTER-GROWN PLANTS

The effect of root temperature on growth and yield of rockwool-grown t omato plants infected with Phytophthora cryptogea was investigated. Me asurements of shoot and root growth were taken at high (25-degrees-C) and low (15-degrees-C) root temperatures during the generative phase o f growth. The growth of roots of healthy and P. cryptogea-infected tom ato plants in rockwool blocks was higher in plants grown with roots at 25-degrees-C than at 15-degrees-C after 60 days and a similar effect was found in slabs after 98 days. Under sub-optimal conditions for gro wth the disease became severe when root temperatures were low. Growth of roots was greatest when roots were maintained at a high temperature in combination with an ambient air temperature of c. 15-degrees-C and the response was greater in cv. Counter than cvs Calypso and Marathon . Water-soluble carbohydrates of roots were higher in those produced i n blocks than slabs and were reduced by infection compared to healthy plants with roots at 15-degrees-C and 25-degrees-C. Reduced transpirat ion rates were found 17 days after inoculation in symptomless plants g rown at a root temperature of 25-degrees-C. Infection, regardless of t he temperature of the roots or cultivar, led to reduced stem growth. T he plants grown at 25-degrees-C were taller than those with a root tem perature of 15-degrees-C. After 9 wk of harvest, the cumulative fruit yields in infected cvs Counter and Calypso grown at 25-degrees-C were comparable to that in healthy plants grown at either temperature and c umulative fruit numbers followed a similar pattern. High root temperat ures led to delayed fruit ripening between weeks 3 - 10 and a larger n umber of unripe fruit. The weight of unripe fruit from infected plants grown at 25-degrees-C at the terminal harvest was higher than from he althy plants with roots maintained at 15-degrees-C.