Research report from OSU's North Willamette Research and Extension Center
Delbert Hemphill
OSU Dept of Horticulture
Mary Powelson
OSU Dept of Botany and Plant Pathology
Introduction
Limited fungicide options and the tentative state of continued registration of vinclozolin for snapbeans require development of additional management strategies for suppression of white mold (Sclerotinia sclerotiorum). One encouraging technique for suppression is to eliminate or reduce spring tillage and leave a straw mulch derived from a cover crop. In 1993, weed emergence was greatly reduced and white mold incidence was reduced nearly 98 percent when beans were planted through a barley cover-crop residue. Snapbean yields were not affected by the cover-crop residue. This mold reduction may have been due to the presence of the barley residue acting as a physical barrier between the white mold apothecia and the bean canopy. In addition, several compounds that have antifungal properties have been isolated from both cereal and cole crop residues. Crop residues may also influence the survivability of sclerotia by stimulating soil microbes which act as predators of sclerotia. Alternatively, actively growing cover crops may induce germination of the sclerotia before bean planting and reduce survivability of the sclerotia.
Integrated management of snapbean diseases depends on the availability, understanding, and potential application of several concurrent strategies. Both white and grey mold are diseases that must be effectively controlled. Our past research has indicated that both white and grey mold development may be affected by changes in tillage systems, vegetative management, and the preceding cover crop or cash crop.
The objective for 1997 was to evaluate the impact of a cover crop and the preceding vegetable crop on the incidence of white mold and grey mold in snapbean.
Methods
A field was divided into 15 60 foot plots of 'Jubilee' sweet corn, 'Gem' broccoli, or summer fallow during summer 1996. White mold sclerotia were buried in mesh bags in each plot. Sclerotia survival was extremely low. In the late summer of 1996, these plots were flailed and disked and then split by a winter fallow treatment or a cover crop of triticale plus Austrian winter pea. Treatment combinations were replicated five times. In late May 1997, after taking samples for biomass, the cover crops were mowed and disked several times, and the plots were seeded to Oregon 91 beans, with three rows/5-foot bed. No herbicides were used. A total of 160 pounds N/acre was applied to the bean crop, which along with the high seeding rate, created favorable conditions for development of white mold. Laboratory-grown sclerotia were applied to 9-square-foot sections at the center of each plot. White and grey mold ratings were made on 18 September, and plots were harvested on 24 September, 1997.
Results
Greater cover-crop/weed biomass accumulated on plots which had been in broccoli or summer fallow compared to those which had been in sweet corn (Table 1). Although herbicide residue may have played a role in this effect, the most likely explanation is that corn residue interfered in the drilling of the cover crop. Sweet corn dry biomass was 6.3 tons/acre compared to 2.2 tons/acre for broccoli. Bean yield and number of plants/plot did not vary significantly with previous crop/cover crop.
The number of plants with white mold also did not vary with treatment although there was a tendency for higher white mold incidence in plots planted into cover crop residue. The number of infected pods recovered per plot also tended to be higher following a cover crop. The previous vegetable crop had no influence on white mold. Expressed as a percentage of the total number of plants/plot, treatments did not affect white mold. The cover-crop residue obviously did not provide a barrier between sclerotia on the soil surface and the bean plants. This may be because the sclerotia were applied after planting and may not have remained in or on the residue. It is also doubtful that the white mold incidence was related to sclerotia applied to the plots: incidence did not appear to be higher in the portion of the plot that had sclerotia applied. The high plant density, rank growth, and over-irrigation probably caused the high incidence of white mold in this study. A straw residue on the surface might actually have favored white mold development by contributing to greater surface moisture and humidity in the plant canopy.
The number of plants infected with grey mold was decreased in plots that had been in broccoli the previous summer and tended to be increased in plots with cover crop residue. The number of infected pods per plot also tended to decrease following broccoli and to increase following the cover crop. The percentage of plants affected by grey mold was more than halved following broccoli.
Table 1. Main effect of previous vegetable crop and winter cover crop on yield, white mold incidence, and grey mold incidence in snapbean, NWREC, 1997. Treatment Cover dry Yield White mold incidence Grey mold incidence mass, g/m2 tons/acre # plants % plants # pods # plants % plants #pods ________________________________________________________________________________________ 1996 crop Summer fallow 302 11.4 17.3 22.2 29.5 8.8 12.3 10.2 Sweet corn 180 13.1 19.5 29.2 22.1 8.3 12.3 9.4 Broccoli 245 11.3 19.5 28.2 20.9 3.9 5.4 4.7 Significance * NS NS NS NS * * NS Cover crop None (weeds) 89 12.0 14.6 20.3 19.1 5.5 8.4 6.3 Triticale/pea 362 11.8 23.2 32.7 29.3 8.5 11.6 9.9 Significance ** NS NS NS NS NS NS NS **,*,NSSignificant at 1% and 5% levels, and non-significant, respectively.