Screening Brassicaceous Plants as Biofumigants for Management of Rhizoctonia solani AG1-IA

Brassicaceae plants rich in glucosinolates have been used as biofumigants for management of Rhizoctonia solani (teleomorph: Thanatephorus cucumeris) and other soilborne pathogens. Efficacy of brassica plant tissue has mainly been attributed to toxic isothiocyanates released upon hydrolysis of glucosinolates. Management of R. solani AG 1-IA, the causal agent of sheath blight in rice (Oryza sativa), using biofumigation, is promising but needs more validation. Biofumigation activity of nine Brassicaceae plants and two other related species were evaluated in vitro with soils from Texas, Arkansas, or Mississippi. All plants evaluated significantly suppressed the mycelium growth of R. solani AG 1-IA. Mustard (Brassica juncea) cultivars (‘Brand 199’, ‘Ruby Streak’, ‘Florida Broadleaf’, and ‘Green Wave’) consistently provided the greatest (>90%) mycelial inhibition, while sunn hemp (Crotalaria juncea) and Chinese cabbage (B. rapa) had the least suppressive effect. B. juncea ‘Red Giant’ and ‘Sheali Hong’, turnip (B. rapa), kale (B. oleracea), and arugula (Eruca sativa) showed intermediate efficacy or were inconsistent. Effects of soil pasteurization and plant tissue amendment rates were examined with B. juncea Brand 199 and Texas soil. Inhibition of mycelial growth became greater with increasing plant amendment rates up to 3.2% (wt/wt) in the soil. Soil pasteurization almost completely suppressed the release of allyl isothiocyanate (AITC). The nonpasteurized soil amended with 0.5% (wt/wt) of the plant material released 96% more AITC than the soil amended with 0.25% (wt/wt) of the plant material. The highest levels of AITC release were observed at 12 and 24 h after soil amendment, with 0.25 and 0.5% (wt/wt) of the plant material, respectively. Antifungal effects of B. juncea are attributed to dose-dependent production of volatile AITC and could be used for managing rice sheath blight caused by R. solani AG 1-IA.

Soil Pasteurization and Mycorrhizal Inoculation Alter Flower Production and Corm Composition of Brodiaea laxa `Queen Fabiola'

The ornamental flowering bulb Brodiaea laxa Benth. `Queen Fabiola' was grown with or without arbuscular mycorrhizal fungal (AMF) inoculum in pasteurized or nonpasteurized soil to determine if inoculation altered flower and corm production. The first growing cycle after planting, mycorrhizal inoculation decreased the days to anthesis and increased the number of flowers produced per inflorescence and flower longevity. It also affected the quality of the daughter corm, which influenced flowering the following year. Inoculated plants produced larger daughter corms and more cormels than uninoculated plants, and allocated more biomass to the corms than the cormels, which lowered the average weight of the cormels. Corms produced by inoculated plants also had higher concentration of nitrogen, potassium, zinc, and nonreducing sugars than those produced by uninoculated plants. The beneficial effects of AMF inoculation on flowering and corm/cormel production were generally increased by soil pasteurization. The results indicate that mycorrhizal inoculation may enhance commercial cut flower and corm production of this crop.

Elucidation of the Microbial Complex Having a Causal Role in the Development of Apple Replant Disease in Washington

Systematic studies were conducted to elucidate the role of different soil microbial groups in the development of apple replant disease. Populations of targeted microorganisms were reduced by the application of semiselective biocides and soil pasteurization. Bacteria were not implicated in the disease, because application of the antibiotic chloramphenicol reduced soil populations of bacteria but failed to improve growth of apple transplants, while enhanced growth was achieved at pasteurization temperatures that did not alter attributes of the bacterial community recovered from apple roots. Populations of Pratylenchus penetrans were below the damage threshold level in eight of nine orchards surveyed, and nematicide applications failed to enhance apple growth in four of five replant soils tested, indicating that plant parasitic nematodes have a minor role or no role in disease development. Application of the fungicide difenconazole or metalaxyl enhanced growth of apple in all five replant soils, as did fludioxinil in the two soils tested. Soil pasteurization enhanced growth of apple and resulted in specific changes in the composition of the fungal community isolated from the roots of apple seedlings grown in these treated soils. Cylindrocarpon destructans, Phytophthora cactorum, Pythium spp., and Rhizoctonia solani were consistently isolated from symptomatic trees in the field and were pathogenic to apple. However, the composition of the Pythium and Rhizoctonia component and the relative contribution of any one component of this fungal complex to disease development varied among the study orchards. These findings clearly demonstrate that fungi are the dominant causal agents of apple replant disease in Washington state.