Vegetable Crops

Fungi to Control White Mold on Dry Beans

White mold, caused by Sclerotinia sclerotiorum, is one of the most destructive diseases of bean in temperate regions, and is almost always present in bean crops grown under irrigation. Chemical sprays can be effective in controlling the disease, but do not impact the amount of spores released from the soil-borne sclerotia. Many fungi are known to be mycoparasites or antagonists capable of attacking sclerotia or mycelium of white mold, and have potential as biocontrol agents of S. sclerotiorum.

Five fungal species, including four mycoparasites and one antagonist, were isolated from bean fields in western Canada and evaluated in field experiments in Alberta, Canada for their effectiveness in controlling white mold. The mycoparasites were Coniothyrium minitans, Talaromyces flavus, Trichothecium roseum, and Trichoderma virens, and the antagonist Epicoccum purpurascens. Spore suspensions of each fungus were sprayed onto bean plants two or three times during the early bloom to mid-bloom period. 

Incidence of white mold was significantly reduced by all biocontrol agents. C. minitans and E. purpurascens were the most effective agents, reducing the proportion of plants infected by an average of 56 and 43%, respectively. C. minitans was the only biocontrol agent recovered consistently from sclerotia and diseased seed present in harvested samples. Fifty-nine percent of sclerotia of S. sclerotiorum that were collected from harvested seed were infected by C. minitans in one year and 20% were infected in another year. This study suggests that C. minitans is the most promising of the five indigenous fungi for control of white mold of dry bean under Canadian prairie conditions. 

Source:

Huang, H. C., E. Bremer, R. K. Hynes, and R. S. Erickson. 2000. Foliar application of fungal biocontrol agents for the control of white mold of dry bean caused by Sclerotinia sclerotiorum. Biol. Control 18(3):270-276.

Sweet Alyssum Good for Cabbage Caterpillar Parasitoids

Nectar sources for adult parasitoids helps conserve naturally occurring or augmented populations of these wasps, improving control of the target pest. Few plants are in bloom during much of the cabbage-growing season in northern Florida, so providing such plants may enhance control. Two plants that had been identified as suitable for parasitoids were wild mustard and wild carrot. However, wild mustard is considered a weed by local growers, and wild carrot may not flower rapidly enough. Sweet alyssum (Lobularia maritima) is a non-weedy, hardy winter annual that flowers rapidly and attracts large numbers of parasitic wasps, and is included in many "insectary seed blends" available commercially.

The effects of sweet alyssum flowers on the longevity of two augmentatively-released parasitoids, the braconid wasp Cotesia marginiventris (for cabbage looper control) and the ichneumonid wasp Diadegma insulare (for diamondback moth control) were studied in a greenhouse experiment. C. marginiventris and D. insulare survived nearly 5 and 13 times longer, respectively, when provisioned with honey or with sweet alyssum than with water alone. Of course, in the real world survival might be less due to more harsh environmental conditions.

D. insulare visited sweet alyssum flowers in preliminary studies in an experimental field plot and commercial cabbage field. Future studies will determine if this will translate into improved biological control by either of these two wasps. Sweet alyssum will be planted along field margins, since that's where diamondback moth populations are highest (especially in growers fields that use a collard trap crop technique in which collards, a favored host plant, is planted on the edges of the cabbage field). When planted from seed in October, sweet alyssum flowered as soon as three weeks after planting, and the flowers persisted for 5 to 6 months. This may be one way to improve biological control by augmentatively-released natural enemies of caterpillar pests by increasing adult parasitoid longevity during times when few wild plants are in bloom.

Source:

Johanowicz, D. L. and E. R. Mitchell. 2000. Effects of sweet alyssum flowers on the longevity of the parasitoid wasps Cotesia marginiventris (Hymenoptera : Braconidae) and Diadegma insulare (Hymenoptera : Ichneumonidae). Florida Entomologist 83(1):41-47


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