6.24
CHEMICAL CONTROL OF STEM CANKER, CAUSED BY RHIZOCTONIA SOLANI ON POTATOES IN SOUTH AFRICA

JC DU PLESSIS and L MEYER

Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, South Africa

Background and objectives
Stem canker and black scurf, caused by Rhizoctonia solani, are serious diseases of potatoes in South Africa. Integrated pest management is commonly practised to reduce disease incidence, but the primary control strategy remains chemical seed and soil treatment. Although widely used, fungicides registered against stem canker in South Africa provide inconsistent control. Field trials were therefore conducted to compare eight candidate fungicides for their effect on stem canker in infested and uninfested soil.

Materials and methods
Fludioxonil, iprodione+imazalil, mepronil, pencycuron, thiabendazole and tolclofos-methyl+thiram were applied to potato seed (cv. BP1) with different infection levels (75 and 15% of tubers infected), whereas dichlorophen and quintozene were incorporated into the soil. Seed was planted in MBr-fumigated, solarized and untreated (control) soil at two localities and the incidence of stem canker assessed after 5 weeks.

Results and conclusions
As expected, disease incidence was highest in plants emerging from seed with high levels of infection. Plants from control and soil treatments showed significantly more stem canker lesions than those from treated seed. All seed treatments reduced disease, but only tolclofos-methyl+thiram exhibited complete control at high infection levels. MBr-fumigation and solarization had no effect on disease in plants from seed with a low level of infection. However, with severely infected seed, a significant reduction in disease was evident in solarized plots. Solarization also enhanced the efficacy of chemical soil treatments, although not to the same level as was achieved with seed treatment. This may be due to induced soil suppressiveness towards the pathogen [1] or because of stimulation of the internal resistance of the plants owing to increased availability of nutrients, which is characteristic of solarization. Soil solarization in combination with seed treatment could therefore be an effective way of controlling stem canker on potatoes in South Africa. The efficacy of the seed treatments supports data that stem canker originates from seedborne inoculum [2].

References
1. Davis JR, 1991. Soil Solarization. FAO, Rome, pp. 28-36.
2. Mulder A, Turkensteen LJ, Bouman A, 1992. Netherlands Journal of Plant Pathology 98, 103-114.