Control of Soilborne Fungi with Biofumigation

Control of Soilborne Fungi with Biofumigation

Summary

The use of two brassica cover crops as biofumigants to control soilborne fungi and Fusarium wilt of watermelon is being evaluated. In spring 2004, Brassica juncea cv. Cutlass and B. napus cv. Dwarf Essex were disked into the soil at flowering. Half of the biofumigation plots were covered with plastic mulch immediately after disking and the remaining plots one month later. Other treatments included a fallow nontreated control and the application of methyl bromide. Inoculum densities of Pythium spp., Rhizoctonia solani, fluorescent Pseudomonas spp., Sclerotium rolfsii, and Fusarium oxysporum were quantified before and after disking. Forty days after the incorporation of the cover crops, seedless watermelon cv. Tri-X 313 was transplanted into experimental plots. Severity of Fusarium wilt was assessed and yield data collected. Isolates of Fusarium oxysporum recovered from watermelon vines are being tested for pathogenicity and race.

Objectives/Performance Targets

1. 1. Evaluate the effectiveness of biofumigation in the control of Fusarium wilt of watermelon, compared to control with methyl bromide. 2. Determine the best time lay plastic after incorporating green manure. 3. Quantify inoculum density of Fusarium oxysporum, Rhizoctonia solani, Pythium spp. Sclerotium rolfsii, and flourescent pseudomonas in the soil before and after biofumigation. 4. Determine glucosinolate concentration in roots and shoots of the brassicas at the time of incorporation. 5. Quantify glucosinolate breakdown products in the soil after brassica incorporation.

Accomplishments/Milestones

1. A month after the incorporation of mustard and canola, watermelon seedlings (Citrullus lanatus) cv. Tri-X 313 were transplanted 3 feet apart into raised plots, 80 feet long and 6 feet wide. Despite having initial lower Fusarium inoculum density at planting, methyl bromide plots had the highest disease rating (28% leaf surface area wilted) at the end of the season. Biofumigated plots planted with ‘Cutlass’ and laid with plastic immediately after disking had the lowest disease severity (17%). Fusarium oxysporum isolates recovered from symptomatic wilted vines are being screened for pathogenicity and race. Eight of 24 isolates induced 50 – 100 % wilting on cv. Black Diamond, which is susceptible to all races. Currently, these eight Fusarium oxysporum f. sp. niveum isolates are being screened to determine race using the differential cultivars Black Diamond, Charleston Gray, and Calhoun Gray. At harvest weight and number of marketable fruit were not significantly different between any of the treatments.

2. Plastic was laid in half the number of canola and mustard plots immediately after plants were disked-in to see if soil residence time of isothiocyantes would be increased. Isothiocyantes were detected in both mulched and non-mulched plots. We are still evaluating the type of isothiocyantes in each type of plot.

3. Inoculum densities of Fusarium oxysporum, Pythium spp., Sclerotium rolfsii, Rhizoctonia solani and flourescent pseudomonads, were quantified before and after disking. To determine the effect of the treatments on microbial counts, inoculum density was determined from soil samples taken a week before disking and at planting time (40 days after disking). Populations did not differ among treatments before disking. At planting, inoculum densities of Fusarium oxysporum and Pythium spp. were significantly lower in the methyl bromide treatment compared to the biofumigation treatments. Fusarium oxysporum counts in methyl bromide plots were not significantly lower than those of control plots. In contrast, control plots had significantly higher Pythium spp. counts than methyl bromide treated plots. Both Pythium spp. and Fusarium oxysporum counts in biofumigation plots were not significantly different from each other or to the control. Rhizoctonia and Sclerotium rolfsii inoculum densities and counts of flourescent pseudomonads were not significantly different between any of the treatments at planting.

4. In 2004 and 2005 the average mustard biomass at disking was 4754 and 8489 kg/ha, while that of canola was 3984 and 9659 kg/ha. At disking approximately 50% of canola and 10% of mustard plants had flowered in 2004, compared to approximately 90% of canola and 50% of mustard plants in 2005. Five glucosinolates, i.e., R-2-hydroxy-3-butenyl, 2-phenylethyl, 3-butenyl, 2-propenyl, and S-2-hydroxyl-3-butenyl were identified in mustard cv. Cutlass (Brassica juncea), whereas three glucosinolates, i.e., 3-butenyl, benzyl, and 2-phenylethyl, were identified in canola cv. Dwarf Essex (B. napus). The predominant glucosinolate in the roots of both canola and mustard was 2-phenylethyl, which is bioactive. Root tissue of both cultivars had a higher concentration of 2-phenylethyl than shoot tissue. Comparing the two cultivars, mustard had a higher concentration of 2-phenylethyl, 21.7 µm/g root tissue, while that of canola was 4.5 µm/g root tissue. The rest of the glucosinolates in canola were each less than 1 µm/g while those in mustard ranged from 0.02 – 2.87 µm/g.

5. Soil samples were taken at 0-15 cm deep, six different times after the plants were incorporated to identify glucosinolate breakdown products. No isothiocyanate was detected at sampling time zero i.e., plant tissue incorporation time. Two isothiocyanates, 2-phenylethyl or benzyl, were detected at 6, 24, and 48 hr after disking. Two-phenylethyl isothiocyanate was detected in canola and mustard plots, but benzyl isothiocyanate was only detected in canola plots. Isothiocyanates were not detected in soils at the fifth and sixth sampling times, 12 and 26 days respectively after disking.

Impacts and Contributions/Outcomes

We have confirmed that the canola and mustard cultivars used in this study contain bioactive glucosinolates when grown under field conditions in the southeastern coastal plain. In addition, we confirmed that roots of these cultivars contain much higher concentrations of a bioactive glucosinolate per gram of tissue than shoots do. More importantly, these glucosinolates are converted to isothiocyanates after the cover crops are disked into soil. Isothiocyanates were detected for 2 days after incorporation. It remains to be determined if this length of time is enough to control Fusarium wilt of watermelon and other soilborne pathogens. We also still need to correlate the concentrations of the different isothiocyanates with populations of soilborne pathogens.

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