Brassica Mulches and Meal Control Fungal Wheat Pathogens in vitro and Take-all Disease in Soil

Soilborne pathogens have an economic impact on crops throughout the world. Many survive saprophytically and therefore are difficult to control. Soil fumigation and seed treatments are sometimes effective. However, they are economically impractical for many crops. Crop rotation/alternative crops are often a practical, economical, and successful means of controlling soilborne pathogens. Recent studies have revealed that Brassica spp. Inhibit a wide array of plant pathogens. Decomposing species of Brassica release glucosinolates, which are converted to isothiocyanates (ITC) by the enzyme myrosinase. The ITC compounds inhibit microorganisms. The present study had three objectives. The first was to evaluate the effect of Brassica spp. On growth of Gaeumannomyces graminis var. tritici (Ggt), Fusarium oxysporum, F. solani, and F. graminearum, and to determine if inhibition was fungicidal. The first in vitro bioassay was designed as a 3 x 3 factorial in a completely randomized design (CRD) with three isolates of Ggt and three mulch treatments, with 3 replicates. The second bioassay was a 6 x 6 factorial with six fungi [three isolates of Ggt (A2, WX, and 211.1), F. solani, F. oxysporum, and F. graminearum] and six mulch treatments (B. juncea ‘Florida Broadleaf’ mustard mulch, B. juncea mustard seed meal, B. napus ‘Dwarf Essex Rape’, B. napus canola, wheat, and no mulch) in a CRD with three replicates. Plant tissue was placed in 490-cm³ glass jars covered by inverted Petri dishes containing potato dextrose agar (PDA) with a 1-day-old fungal plug. Colony diameters were recorded for eight days. All fungal pathogens tested were inhibited by B. napus and wheat mulch, but fungal growth resumed when the mulches were removed. No growth was recorded for fungi exposed to B. juncea mulch and seed meal, where inhibition was fungicidal. The second objective of the present study was to determine the amount of allyl isothiocyanate (AITC) needed to inhibit growth of the take-all pathogen. The tests were designed in a CRD with five rates of AITC or five rates of B. juncea seed meal. Gaeumannomyces graminis var. tritici isolate WX was exposed to AITC, a dominant isothiocyanate compound found in Brassica spp. Treatments of pure AITC and AITC from B. juncea mustard seed meal (mixed with water) at 0, 0.01, 0.02, 0.03, and 0.04 g were evaluated for the effects on Ggt growth. Petri plates with PDA and 1-day-old fungal plugs were inverted over 490- cm³ glass jars with AITC or seed meal treatments for 5 h. Headspace concentrations of pure AITC and AITC volatized from B. juncea seed meal were recorded after 1 and 2 h; mycelial diameter was measured for 9 days to determine suppression or death of the fungus. Mycelial growth was inhibited by all B. juncea seed meal treatments. The concentrations of AITC produced by B. juncea seed meal that inhibited 50 and 90% of mycelial growth were 0.43 and 0.80 μmol∙liter^-1, respectively. Inhibition attributable to pure AITC alone was higher than that achieved by B. juncea seed meal. The third objective of the present study was to determine if incorporating B. juncea mustard mulch into soil containing Ggt-infested wheat crowns would reduce take-all in a subsequent wheat crop, and to evaluate phytotoxicity of Brassica mulch to wheat seedlings. The experiment was designed as a factorial in a split plot with two rates of Ggt (present or absent), two rates of B. juncea ‘Florida Broadleaf’ mulch (present or absent), and ten replicates. The main plot was Brassica mulch; the sbu-plot was Ggt. There were ten plants per replicate and the study was repeated. For the initial crop, wheat seeds were planted into soil in plastic containers with and without inoculum of Ggt, isolates A2 and WX, and maintained in a growth chamber 18˚C. After 28 days, shoots were excised and wheat roots were left in soil. Soils with healthy and diseased roots were mulched with ‘Florida Broadleaf’ mustard for five days; mulch was not applied to controls. For the ‘final crop’ wheat seeds were replanted into soil; take-all severity, shoot height, and percent germination were measured after 28 days. The main effects of mulch and Ggt, and the interaction were significant for disease severity in two trials. Treatment of soil containing Ggt-infected roots with Brassica mulch significantly reduced take-all in the subsequent wheat planting in two tests. In one test, height of seedlings was reduced in soil with Brassica mulch and seedling germination was reduced with Brassica mulch regardless of the presence of Ggt.