Control of Soilborne Fungi with Biofumigation

Control of Soilborne Fungi with Biofumigation

Summary

In 2004 and 2005, mustard (Brassica juncea) cv. Cutlass and canola (B. napus) cv. Dwarf Essex were evaluated as biofumigation crops for the control of Fusarium wilt of watermelon and soilborne fungi. Treatments included disking flowering canola or mustard plants and laying black polyethylene mulch at incorporation or 1 month later, methyl bromide, and a nontreated control. Prior to incorporation in 2004 and in 2005, the inoculum density of Pythium spp. was significantly higher in some of the canola and mustard plots compared to those in non-planted treatment plots. We hypothesized that non-significant differences in pathogen population in 2004 among amended and non-amended control might have resulted in part from low plant biomass incorporated. Therefore, in 2005, we doubled the seeding rate for both canola and mustard, resulting in incorporating greater than 100% more plant biomass than in 2004. In addition, instead of quantifying soilborne fungi 50 days after incorporation, in 2005 we quantified populations 22 days after incorporation. Prior to incorporation in 2005, Pythium spp. population densities in canola and mustard plots were significantly higher than in non-planted control plots. Results from 2005, also showed that isothiocyantes were detected erratically in the soil up to 2 days after incorporation. After incorporation in 2005, amended plots had significantly higher populations of F. oxysporum and Pythium spp. than the methyl bromide treatment, but populations were not significantly different from the control. In 2005, Pythium spp. inoculum density was significantly lower in mustard-amended plots covered with polyethylene mulch at incorporation than in plots covered 1 month later. No differences in populations of Rhizoctonia solani were detected in 2005. In both years, no significant differences in yield or Fusarium wilt were detected.

Objectives/Performance Targets

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

Accomplishments/Milestones

There were no significant differences in percentage of wilted plants or yield of fruit among treatments.

In 2005, the average mustard biomass at incorporation on 5 April was 8489 kg/ha, while that of canola was 9659 kg/ha. Biomass in 2005 was 1.8 and 2.4 times greater than in 2004 for mustard and canola, respectively. At incorporation approximately 90% of canola and 50% of mustard plants had flowered. Most of the non-flowering plants had unopened flower buds.

In 2005 prior to incorporation, canola and mustard plots had significantly higher population densities of Pythium spp. than fallow plots. After cover crops were incorporated, Pythium spp. significantly increased in canola-amended plots that were not covered with polyethylene mulch immediately after incorporation compared with population densities before incorportation. Pythium spp. did not significantly change in mustard-amended plots not covered with polyethylene mulch immediately after incorporation. In contrast, Pythium spp. decreased significantly, compared with population densities before incorportation, in canola- and mustard-amended soils covered with polyethylene mulch immediately after incorporation. However population of Pythium spp. in canola and mustard-amended soils covered with plastic immediately after incorporation were not significantly lower than in control nonamended plots. Pythium spp. inoculum density in methyl bromide plots was significantly lower than in the other treatments.
Fusarium oxysporum populations did not differ significantly among treatments prior to incorporation. After treatments were applied, F. oxysporum significantly decreased in methyl bromide and nonamended control plots. F. oxysporum did not decrease in mustard plots covered with polyethylene mulch immediately or a month after incorporation. F. oxysporum also did not decrease in canola-amended soils covered with polyethylene mulch a month after incorporation. However there was a significant increase in F. oxysporum density in canola-amended soils covered with polyethylene mulch immediately after incorporation. At the end of the season, there were no significant differences in the inoculum density of F. oxysporum between soils treated with methyl bromide compared to canola- and mustard-amended soils. However, at the end of the season, nonamended control soils had a significantly lower population of F. oxysporum than the other treatments.

In 2004 and 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 (L.) Czern), whereas two glucosinolates i.e., 3-butenyl, benzyl, and 2-phenylethyl were identified in canola cv Dwarf Essex (B. napus L.). The predominant glucosinolate in the roots of both canola and mustard was 2-phenylethyl. Quantitatively, root tissue of both cultivars had a higher concentration of 2-phenylethyl than shoot tissue. Glucosinolate quantities from 2005 samples have not yet been calculated.

In both years, isothiocyanates were not detected at sampling time zero i.e., plant tissue incorporation time. Also in both years, we detected 2-phenylethyl and benzyl isothiocyante in the soil erratically from 6 hours up to 2 days after incorporation.

Impacts and Contributions/Outcomes

Despite increasing plant biomass we generally did not achieve significant reductions in soilborne fungi populations compared to the nonamended control. Soil temperature at the time of incorporation might be the critical factor in enhancing the release of fungitoxic chemicals from the biofumigation plants. Gamliel and Stapleton (1990) reported that volatiles from cabbage residues in heated soils reduced the viability of P. ultimum and S. rolfsii, compared with those exposed to volatiles from cabbage residue in unheated soils. When these volatiles were analyzed, generally they did not find isothiocyanates in nonheated soils. A biofumigation experiment carried out in the summer would confirm this.

Gamliel, A., and Stapleton, J.J. 1990. Characterization of antifungal volatile compounds evolved from solarized soil amended with cabbage residues. Phytopathology 83:899-905.

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