Disease cycle of seed-borne Rhizoctonia-disease

Studies on etiology and control of the stem canker and black scurf diseases caused by Rhizoctonia solani in Finland have been carried out at University of Helsinki, Department of Applied Biology, since 2003. One of the studies repeated during three growing seasons was carried out to provide a detailed characterization of the disease cycle. Because the diseases caused by R. solani can be soil- and seed-borne, the experiments were placed in an R. solani free field and established using seed tubers infested with black scurf, so to be able to focus on the seed-borne infection. Only the young below-ground parts of the potato plants were detectably infected. Four phases could be distinguished during the disease cycle, all of which were characterized by different phenotypic features and consequences on yield. The first phase included infection of the tips of sprouts prior to emergence. It resulted in delayed emergence, fewer stems and retarded development of canopy, and reduced yield and starch content in tubers. The second phase included infection of stolon tips 2-3 weeks after emergence of plants. During this phase the stems were no more damaged. Infection and death of stolon tips resulted in shorter stolons than observed in uninfected plants. Consequently, infected plants had a tight tuber nest in which tubers were misshaped, furrowed and highly variable in size. The third phase of the disease progression, 3 to 4 weeks after emergence, resulted in damage to the youngest tubers. They suffered from scabby, often sunken surface areas and holes, which were usually situated at the sprout-end of the tuber. In some varieties, cracking of tubers was also observed. The fourth and last phase was defined by the development of sclerotia (black scurf) on tubers. This phase was initiated by the time of maturation in all below-ground parts of the potato plant. It was triggered also by chemical haulm killing at ca. 2 weeks after the treatment. Between the aforementioned, active phases of disease progression, R. solani seemed to be inactive, residing as a tight, brown mycelium on stem bases, stolons and roots. The mycelium also formed typical fruiting bodies under moist conditions. These studies were carried out with financial support from the private sector, advisory organisations, Ministry of Agriculture and Forestry and University of Helsinki.

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