Studies on the biology and control of cavity spot of carrots

Using a selective medium of corn meal agar with pimaricin and rifamycin, isolations from asymptomatic periderm of carrots grown on experimental plots or in commercial crops most frequently yielded the fast-growing species Pythium intermedium, P. sylvaticum or P. ultimum. In contrast isolations from cavity spot lesions mostly produced the slow-growing species P. violae and P. sulcatum. Following treatment of crops with metalaxyl + mancozeb, few isolations from asymptomatic periderm produced Pythium spp. and generally there was a reduction in the number of cavities. Treatment had little or no effect on size distribution of cavities. Cavity spot incidence was significantly less at higher pH values, fields of pH 8·0 and above producing carrots with little or no disease.