Effect of soil temperature on the pathology of Alternaria radicina on carrots

Abstract

The temperature of muck soil infested with Alternaria radicina Meier, Drechsler, and Eddy was found to have a marked effect upon the severity of root dieback or rusty root of carrots, Daucus carota L. When the surface of steam-sterilized muck soil was artificially reinfested with as few as 50 spores of A. radicina per square centimetre, less than 1% emergence occurred below 18 °C in controlled soil-temperature experiments. In muck soil naturally infested with A. radicina, carrot seedling emergence over the whole range of temperatures, 10–28 °C, was little more than half that obtained in steam-sterilized similar soil. The severity of root dieback increased sharply with decreasing temperature below 18 °C in both artificially reinfested and naturally infested soil. Alternaria radicina was found actively growing on carrots in naturally infested soil at and below 16 °C, but, when similar soil was steam sterilized and artificially reinfested after seedling emergence, A. radicina infected the young carrots in soil up to 26 °C. Alternaria radicina was consistently isolated both from carrots with symptoms of root dieback or rusty root and from the muck soil in which the carrots had been grown and had produced symptoms of root dieback or rusty root. An analysis of variance of the root weight data of the five cultivars of carrots used showed that the effect of soil temperature was independent of the effect of the infested soil on root growth. A multivariate analysis of variance affecting all means of root weight of the five cultivars showed that, in general, any selected number of carrot cultivars would each vary in root weight according to the soil temperature and to the infested soil significantly and independently.