Effects of Pathogen Density, Soil Moisture, and Soil pH on Biological Control of Clubroot in Chinese Cabbage by Heteroconium chaetospira

The effects of soil moisture and pH, and pathogen resting spore density, on the effectiveness of the biological control of clubroot by the fungal endophyte Heteroconium chaetospira was evaluated in greenhouse and field experiments. Conditions favoring disease development included low pH (5.5) and high soil moisture content (80%), with significant reductions in the disease being observed at a higher pH (6.3 and 7.2) and lower soil moisture content (40 and 60%). In greenhouse tests, H. chaetospira effectively controlled clubroot (reducing the disease by 90 to 100%) at pathogen resting spore densities of 104 and 105 spores/g of soil at all soil pHs tested (5.5, 6.3, and 7.2). However, when the resting spore density was 106 spores/g of soil, plants were severely diseased, regardless of treatment, and H. chaetospira had no effect on disease. At a soil moisture content of 40%, disease occurrence was low, regardless of pathogen spore density, but disease was significantly lower in H. chaetospira-treated plants at pathogen spore density of 105 spores/g of soil. At 60% soil moisture content, H. chaetospira significantly could affect at pathogen spore densities of 104 and 105 but not 104/g of soil. At 80% soil moisture content, there was no effect of H. chaetospira at pathogen density. In situ, the soil moisture contents were constantly adjusted to relatively low to moderate (pF 2.2 to 2.4 and pF 2.0 to 2.2) and high (pF 1.6 to 1.8). Other environmental conditions, such as resting spore density and soil pH, were maintained at constant levels. Control plants (not treated with H. chaetospira) showed uniformly high disease levels and proportions of diseased plants across all three moisture treatments (disease index = 72 to 80, proportion of diseased plants 85 to 97%). In the field, H. chaetospira-treated plants at low soil moisture (pF 2.2 to 2.4, plot 1) had 68% disease reduction compared with untreated controls and 49% reduction at moderate moisture pF (pF 2.0 to 2.2, plot 2). There was no effect on disease by H. chaetospira at high soil moisture (pF 1.6 to 1.8, plot 3). Based on our results, H. chaetospira is an effective biocontrol agent against clubroot in Chinese cabbage at a low to moderate soil moisture range and a pathogen resting spore density of 105 (or lower resting spores per gram of soil in situ.

Control of Verticillium Yellows in Chinese Cabbage by the Dark Septate Endophytic Fungus LtVB3

Three hundred forty-nine fungal endophytes were obtained from a total of 1,214 root segments of eggplant, melon, barley, and Chinese cabbage grown as bait plants in a mixed soil made up of samples from different forest soils in Alberta and British Columbia, Canada. Three of the 349 isolates, when inoculated in axenically reared Chinese cabbage seedlings grown in petri dishes, almost completely suppressed the effects of a postinoculated and virulent strain of Verticillium longisporum. Two isolates effective against the pathogen were Phialocephala fortinii, which had been obtained from the roots of eggplant and Chinese cabbage. The third isolate was a dark septate endophytic (DSE) fungus obtained from barley roots. Hyphae of P. fortinii grew along the surface of the root and formed microsclerotia on or in the epidermal layer. Hyphae of the DSE fungus heavily colonized root cells of the cortex. Seedlings grown for 1 week in the presence of the endophytes were then challenged with the Verticillium pathogen. In DSE-treated roots, some of cell walls in the epidermal and cortical layers showed cell wall appositions and thickenings, which appeared to limit the ingress of the pathogen into adjacent cells. Such marked host reactions were not observed in the root cells colonized by P. fortinii. Chinese cabbage preinoculated with the above endophytes and, for comparison, a previously reported disease-suppressive fungal endophyte, Heteroconium chaetospira, were transplanted into the field and disease symptoms were assessed. The DSE could most effectively inhibit the development of Verticillium yellows, with reductions in the percentages of external and internal disease symptoms of 84 and 88%, respectively. The protective values against the disease are extremely high compared with those of other isolates. Most of the DSE-treated plants in the plots achieved marketable quality.