Rhizoctonia foliar blight, caused by Rhizoctonia solani Kühn anastomosis group 1, causes rapid and severe destruction of soybean foliage and pods during hot, wet weather. The objectives of this study were to determine the yield components affected by this disease and whether rating pod damage or foliar damage provides a more reliable method of assessing disease severity relative to impact on yield. Disease severity in the moderately resistant cv. NK S57-11 and the susceptible cv. Buckshot 723 was assessed in field plots in 1996 and 1997 using foliar and pod ratings (0-to-10 scale corresponding to 0 to 100% of tissue affected). Based on results from regression analysis, pod number, seed number, and seed weight per plot decreased as disease severity increased, whereas the proportion of partially filled pods and the weight of 100 seed were not affected. Yield loss appeared to be due primarily to loss of entire pods. Foliar and pod assessments of disease severity correlated positively in 1996 (r = 0.8343) and 1997 (r = 0.5958) for both cultivars, which suggests that either method can be used to identify relative differences among cultivars. However, pod assessments accounted for more variability than foliar assessments under low-disease conditions. Plants exhibiting moderate to severe symptoms of Rhizoctonia foliar blight also retained green stems and pods at harvest, which was evidence of delayed maturity.

The fungus Myrothecium verrucaria (Alb. & Schwein.) (MV), originally isolated from diseased sicklepod (Senna obtusifolia L.), has bioherbicial activity against kudzu and several other weeds when applied with low concentrations of the surfactant Silwet L-77. To more fully understand the initial events of MV infection or disease progression, and to improve knowledge related to its mechanism of action, the effects of MV and its product (roridin A) on kudzu seedlings were examined at the ultrastructural level. Ultrastructural analysis of MV effects on kudzu seedlings revealed a rapid (~1 h after treatment) detachment of the protoplast from the cell wall and plasmodesmata appeared to be broken off and retained in the wall. These symptoms occurred well in advance of the appearance of any fungal growth structures. Some fungal growth was observed after severe tissue degeneration (24 to 48 h after treatment), but this occurred primarily at the extra-cellular location with respect to the kudzu tissues. Kudzu seedlings treated with roridin A, a trichothecene produced by the fungus, exhibited some symptoms similar to those induced by the fungus applied in spore formulations with surfactant. The overall results are the first to report the ultrastructural effects of this bioherbicide on plants and suggest that penetration of a phytotoxic substance(s) in the fungal formulation was facilitated by the surfactant, and that roridin A exerts phytotoxicity toward kudzu.

The shelf-life of a bioherbicide product is an important factor with regard to its commercial potential. The bioherbicidal efficacy of freshly fermented Myrothecium verrucaria (strain IMI 368023) (MV) mycelia formulations and MV mycelia preparations that had been freeze-dried and then stored at −20˚C for 8 years was compared. Two concentrations of each formulation (1.0x and 0.5x) were tested, utilizing bioassays on seedlings of the weed, hemp sesbania (Sesbania exaltata) under greenhouse conditions or in darkness utilizing hydroponically grown seedlings. Freeze drying of freshly prepared MV mycelium produced a light, brownish-colored powder. Efficacy tests of this reconstituted 8-year-old dried material showed that some bioherbicidal activity was lost during long-term storage, i.e., ~20% and ~60% seedling dry weight reduction at the 1.0x and 0.5x rate, respectively. Although plant mortality was greater in the fresh mycelial preparations treatments versus the freeze-dried and stored samples at all time points in the time-course, the stored material still caused >80% mortality, 15 days after treatment. Comparative disease progression ratings also showed a similar trend. Overall results show that freeze-drying MV is a useful method to reduce the bulk and cumbersomeness of storing heavy liquid fermentation product, while retaining bioherbicidal activity. These findings increase the utility of this bioherbicide and offer the potential to use the dried material in soil treatments or in a more concentrated form than attainable via the fermented product.