1PAV, Applied Research for Arable Farming and Field Production of Vegetables, PO Box 430, 8200 AK Lelystad, The Netherlands; 2IPO-DLO, Research Institute for Plant Protection, PO Box 9060, 6700 GW Wageningen, The Netherlands; 3IRS, Institute for Sugar Beet Research, PO Box 32, 4600 AA Bergen op Zoom, The Netherlands

Background and objectives
Rhizoctonia solani is a major problem in sugar beet cultivation on sandy soils in the Netherlands. After frequent sugar beet growing losses may be high. In addition Rhizoctonia disease can be severe after successive cropping of maize or grass. A joint research project between IPO, IRS and PAV investigates the management possibilities to limit the damage by R. solani in arable and vegetable crops. The research focuses on detection in soil and identification in conjunction with a bioassay to predict disease development. The possibilities for decreasing soil inoculum potential are investigated by growing resistant crops or non-hosts and by incorporating crop residues with or without antagonists (biosanitation with Verticillium biguttatum). Disease development and resulting yield and quality losses may be minimized by effective chemo- and biocontrol and by growing resistant varieties.

The aim of this research was to study the host plant specificity of arable, vegetable and green manure crops in greenhouse tests. This was done in three ways: (i) by assessing plant symptoms after inoculation; (ii) by determining inoculum potential of the soil after different hosts had grown for 6-8 weeks; and (iii) by determining inoculum potential of soil after the shoots of different hosts had been incorporated.

Results and conclusions
Plant symptoms: the presence of symptoms on hosts was evaluated after growing in 1-l pots with a mixture of pasteurized sand and peat inoculated with a pathogenic isolate (LU) of R. solani AG 2-2 from diseased sugar beet. All the crops tested showed lesions of R. solani on the roots. Moreover there were also lesions on shoots of cereals, radish and potato tubers. Sensitive hosts with significantly lower yields are black salsify, sugar beet, carrot, lettuce and maize.
Inoculum potential of R. solani AG 2-2 on roots of different hosts: the inoculum potential of Rhizoctonia was evaluated by means of a bioassay with sugar beet. Different crops were grown for 6-8 weeks. The crops were harvested and the soil with the roots was dried out before it was re-wetted in the bioassay. Compared to the inoculum potential of R. solani AG 2-2 after fallow soil without a crop, two levels of inoculum potential may be recognized. Most of the crops maintain a high level in the soil, such as sugar beet, potato, maize, carrot, leek, black salsify, lettuce, ryegrass, marigold, etc. There is a small group of crops which maintain a low level of inoculum potential. The low level group includes radish and fodder radish.
Inoculum potential of R. solani AG 2-2 on shoots of different hosts: application of shoots (as crop residues) stimulated inoculum potential of R. solani in pasteurized and inoculated soil. If shoots are mixed through naturally contaminated soil this stimulates R. solani, but also the antagonistic flora and fauna. The balance between them is variable. Application of V. biguttatum spores onto crop residues resulted in reduced inoculum potential of R. solani in bioassays (biosanitation).

It is concluded that most of the crops are good hosts for R, solani AG 2-2 isolate LU, and only radish or fodder radish are moderate hosts. On fields contaminated with R. solani AG 2-2, sensitive crops should be grown after a period of fallow cropping or after growing radish or fodder radish.

Since R. solani is not limited to the Netherlands, we seek international cooperation with breeding companies and research institutes to get synergism in Rhizoctonia research, in participation in collective ring tests on (transgenic) resistant varieties, optimization of crop rotation and crop residue management, soil receptivity, routine detection and identification methods and bioassays.