5.6.5
RHIZOCTONIA DISEASE OF POTATO (RHIZOCTONIA SOLANI): FUNGICIDAL CONTROL, SUSCEPTIBILITY OF POTATO CULTIVARS AND ADVANCED BREEDING LINES AND INFECTION OF ROTATION CROP PLANTS WITH THE PATHOGEN

PIARA BAINS 1and CLIVE SCHAUPMEYER2

1Crop Diversification Centre North, Alberta Agriculture Food and Rural Development, R.R. #6, Edmonton, Alberta,T5B 4K3 Canada; 2 Crop Diversification Centre South Alberta Agriculture Food and Rural Development SS #4, Brooks, Alberta, T1R 1E6 Canada.

Background and objectives
Rhizoctonia canker, also known as stem canker and black scurf of potato (Rhizoctonia solani Kühn), causes serious economic losses to potato growers in temperate regions of the world. Except in fields under continuous potato production, infected seed pieces are the major cause of disease development [1]. Reducing pathogen levels on seed piece therefore is very crucial for effective control of the disease. Seed piece treatment Fungicides, presently registered in Canada, are not effective in controlling the disease to growers' satisfaction. The objectives of the project were to identify a seed piece treatment fungicide which will effectively control the disease, determine the relative susceptibilities of potato cultivars and advanced breeding lines to R. solani, and determine the pathogenicity of plant species, used in rotation with potatoes, to R. solani.

Materials and methods
Fungicide screening and cultivar susceptibility experiments were conducted at Bon Accord, Alberta, Canada, in a field where potatoes were not planted for 3 years. Fungicide screening experiment: R. solani-infected Yukon Gold tubers were treated before planting with one of the seven fungicides [Captan 50% WP (captan), Easout 10% d (thiophanate-methyl), Maxim 0.33% D (fludioxonil), Maxim 0.50% D, Mertect 45% F (thiabendazole), Rovral 50% WP (iprodione), and Tuberseal 16% D (mancozeb)]. Control treatment tubers had no fungicide application. Cultivar susceptibility experiment: seed tubers of advanced breeding lines (AV 82101-12, F119649-6, V0024-6, V0123-25, V0299-4) and potato cultivars (AC Ptarmigan, Alpha, Amisk, Bintje, Chipeta, Norgold Russet, Norland, Rode Earstling, Russet Burbank, Shepody, White Rose) were inoculated with R. solani by placing the inoculum (a mixture of grounded R. solani sclerotia and talc) on seed tubers immediately before they were covered with soil. Rhizoctonia stem canker severity was determined by recording the length and percentage of stem circumference affected (circumference factor, <25%=1, 25-50%=2, 51-75%=3, >75%=4) for each lesion. Sum of the products of length and circumference factor for lesions of a stem were divided by the product of total stem length and maximum circumference factor. Canker severity was expressed as a percentage of the maximum possible.

Plant species (Beta vulgaris, Brassica compestris, Hordeum vulgare, Pisum sativum, Triticum aestivum, Zea mays) commonly used in rotation with potatoes in Alberta were tested for infection with five potato isolates of R. solani. Plants were grown from seed in soil-less medium mixed with R. solani-infested rye kernels. Roots of the plants were checked for infection.

Results and conclusions
Fifty days after planting both concentrations of Maxim, Captan, and Rovral reduced Rhizoctonia stem canker severity compared to that of control. At harvest, however all the fungicides, except Easout were effective in reducing the stem canker severity and percent stem infection. Captan and both concentrations of Maxim reduced percent incidence of black scurf on tubers, severity of black scurf on tubers, however was only reduced by Maxim 0.5%. The results of various observations taken together suggest that Maxim 0.5% was most effective in controlling the disease. Maxim has recently been granted an emergency registration for control of Silver scurf disease of potatoes (Helminthosporium solani) in some states of USA [2]. None of the cultivars or breeding lines tested was resistant to R. solani. The cultivars and advanced breeding lines showed a continuous range of reactions to the pathogen. AC Ptarmigan, Russet Burbank, and AV82101-12 were more susceptible than Bintje, Shepody, V0299-4, and Amisk. None of the plant species showed infection with potato isolates of R. solani.

References
1. Frank JA, Leach SS, 1980. Phytopathology 70, 51-53.
2. Mattoon M, 1997. Potato Grower 26, 22-24.