Exploring Differential Interactions Between Rhizoctonia solani AG 2-t Isolates and Tulip Cultivars.

Experiments were conducted to explore differential interaction of Rhizoctonia solani AG 2-t isolates on tulip cultivars in soil artificially infested under different experimental conditions. Comparison of residual variances obtained by analysis of variance and by analysis of additive main effects and multiplicative interaction effects (AMMI) showed that open-air experiments should be used for interpretation of isolate by cultivar interaction. In open-air experiments, variability was lower than in greenhouse tests. In the biplot, derived after AMMI-analysis over isolates by years and by cultivars, isolates tended to occur in year clusters, indicating a differential effect of year on disease expression. Three isolates occurred in isolate clusters, thus accounting for a significant year by isolate by cultivar interaction. One cluster consisted of three isolates high in aggressiveness on all tested tulip cultivars, and another cluster consisted of three isolates low in aggressiveness. Greenhouse conditions and inoculum carrier significantly influenced quantitative differential interaction patterns. Isolates grown on oat kernels were more aggressive than those grown on autoclaved perlite particles soaked in liquid malt peptone. Differential interaction of AG 2-t isolates on tulip cultivars does occur, although it cannot yet be disentangled completely.

Adhesive knob formation by conidia of the nematophagous fungus Drechmeria coniospora.

We studied conidiogenesis and adhesive knob formation (maturation) by newly developed conidia of the nematophagous fungus Drechmeria coniospora. Upon conidiogenesis on infected nematodes or during saprophytic growth of the fungus in axenic cultures compact clusters of conidia developed. Less than 10% of such clustered conidia matured; mature conidia were invariably located on the periphery of the clusters. The kinetics and rate of maturation of conidia were studied in in vitro systems and in soil. In both cases adhesive knobs were formed; the rate at which knobs were formed appeared to be determined by the age of the conidia, the temperature and the soil moisture. In addition, knob formation was suppressed at increasing conidial densities. Under favorable conditions, however, over 90% of the conidia matured within a period of 3 days. The rate of knob formation was neither influenced by the presence of nematodes nor by that of exogenous nutrients, which suggests that maturation is an autonomous process. Electron-microscopical analysis indicated that budding of the conidia at the initial stage of maturation occurred simultaneously with the deposition of the sticky, adhesive layer around the wall of the developing knob. The ecological significance of the time- and spatially separated maturation of conidia after conidiogenesis is discussed with respect to survival of the conidia.

Microscopic observations on the interaction of the mycoparasite Verticillium biguttatum with Rhizoctonia solani and other soil-borne fungi.

The mycoparasitic interactions of Verticillium biguttatum with Rhizoctonia solani and with a variety of other soil-borne fungi were investigated in dual cultures. V. biguttatum interacted with various soil fungi by appressed growth along the host hyphae and infrequent penetrations. Intracellular growth and subsequent sporulation, however, only occurred with R. solani, a few binucleate Rhizoctonia and Ceratobasidium spp., and Sclerotinia sclerotiorum. Effective mycoparasitism on sclerotia was restricted to those belonging to R. solani. Electron-microscopic observations revealed that V. biguttatum can penetrate the host cell with infection tubes. This process is probably mediated by enzymatic hydrolysis of the cell wall. Subsequently, trophic hyphae develop within the host cytoplasm, ultimately resulting in death of the host cell.