Phytophthora cryptogea root rot of tomato in rockwool nutrient culture: III. 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 tomato plants infected with Phytophthora cryptogea was investigated. Measurements of shoot and root growth were taken at high (25-degrees-C) and low (15-degrees-C) root temperatures during the generative phase of growth. The growth of roots of healthy and P. cryptogea-infected tomato 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 growth 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 in blocks than slabs and were reduced by infection compared to healthy plants with roots at 15-degrees-C and 25-degrees-C. Reduced transpiration rates were found 17 days after inoculation in symptomless plants grown at a root temperature of 25-degrees-C. Infection, regardless of the temperature of the roots or cultivar, led to reduced stem growth. The plants grown at 25-degrees-C were taller than those with a root temperature 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 cumulative fruit numbers followed a similar pattern. High root temperatures led to delayed fruit ripening between weeks 3 - 10 and a larger number of unripe fruit. The weight of unripe fruit from infected plants grown at 25-degrees-C at the terminal harvest was higher than from healthy plants with roots maintained at 15-degrees-C.