Effects of temperature and length of exposure on the mortality of the carrot cyst nematode, Heterodera carotae

Abstract

A hatchability level of 72% of eggs in Heterodera carotae cysts was notsignificantly affected by keeping the cysts in water at up to 40 C for 2 h, but percentage hatch was strongly reduced by keeping the cysts for 2 h in water at 47.5, 50 and 52.5 C. Keeping soil with cysts of H. carotae with a hatchability before treatment of 27% at temperatures between 32.5 and 45 C for 2 and 4 h did not affect percentage hatch of the eggs in these cysts but longer exposures increased it in two ways and decreased it in one way. This resulted in a positive correlation between percentage hatch and length of exposure up to a smaller maximum hatch the higher the temperature. At still longer exposures percentage hatch was negatively correlated with length of exposure. Whether the decrease is due to mortality of the eggs or, partly, to reversible inactivation remains to be investigated. In contrast to hatching tests with root diffusate, probably almost all eggs will hatch in the field under a host crop. Therefore, only the adverse effect of the heat treatments on hatching is ofpracticalimportance for the estimation of the effect of solarization. To separate it from the effects that increased percentage hatch in the tests, a model was made of the interaction of the separate effects. It was based on linear relationships between log duration of exposure to different temperatures and probit relative increase and decrease of hatching rates, and linear relationships between treatment temperature and log length of exposure required to obtain a given effect of one of the two increasing effects and of the decreasing effect of the treatments. The coefficient of variation of hatchabilities that were affected by the increasing but not by the decreasing effect of the treatments, relative to those predicted by the model, was 7.1% against 7.8% after treatments that had no effect on percentage hatch, whereas the relationship between log length of exposure and probit percent reduction of hatchability by the treatments was linear. The relationship between probit hatchability ( h ) due to the decreasing affect of the treatments and temperature ( T ) during t hours is probit h = a b log t dTr - T in which T is a reference temperature. For T - 10 T r r T = 45 C, a = 11.047, b = 1.44 and d = 0.637 for H. carotae . A good r T r approximation of this relationship is log h= k l t dTr - T with k = 2.453, - T T r l = 0.03735 and d = 0.637. These relationships probably also apply, with T r other parameter values, to other nematode species. To calculate the effect of fluctuating temperatures t dTr - T is replaced in the equations by T Sigma (t dTr - T). T