Biological Control of Bacterial Diseases of Vegetable Crops

Biological Control of Bacterial Diseases of Vegetable Crops

Summary

Bacterial diseases cause significant yield and quality losses in vegetable crops in the North Central region of the U.S. Vegetable producers routinely treat vegetables in the field with copper/EBDC fungicide mixtures to reduce disease development and spread, but such treatments are rarely effective. Resistance to bacterial diseases is not widely available in commercial vegetable varieties, and where resistant varieties exist, new races of the pathogen evolve quickly. Optimal control of bacterial diseases in vegetable crops requires that bacterial populations are never permitted to reach levels that cause a significant level of disease. Our approach is to focus on the production of healthy transplants with negligible bacterial pathogen loads, followed by field applications of biological control agents or low-risk synthetic compounds in an integrated disease management program.

We showed previously that bacterial diseases are reduced in seedlings produced in transplant plug mixes containing composted pine bark, compared with traditional mixes prepared with sphagnum peat. This reduction in disease is thought to be the result of the induction of systemic resistance (ISR) by rhizosphere microorganisms in compost used. Only two of 50 batches of compost-amended planting mixes tested consistently suppressed bacterial spot of radish and tomato. More than 600 bacterial strains have been recovered from the rhizosphere of radish seedlings grown in the two most ISR-active batches of compost-amended transplant mix. Seventy-one of those strains were able to induce ISR in radish bioassays in one of three tests but none induced ISR consistently.

Trichoderma hamatum 382 (T382) has been identified as the most effective fungal biocontrol agent, although it also does not always induce resistance to disease in the radish bioassay. By comparison, the synthetic plant activator Actigard induces systemic acquired resistance consistently when applied to the foliage of tomato plants or radish. Binary inoculants, consisting of T382 and one of the 71 ISR-active bacterial strains, and 21 other similar strains received from other laboratories, are presently being screened for activity on radish. The most efficacious combinations will be tested on tomato, first in greenhouse and thereafter in field bioassays in 2000.

We have identified two strains of bacteria (Bacillus subtilis OSU140 and Paenibacillus macerans OSU 142) that are antagonistic to the bacterial spot pathogen (Xanthomonas campestris pv. vesicatoria) in the laboratory and greenhouse. These strains, as well as three other Bacillus spp., Actigard and standard bactericides were evaluated for control of bacterial spot of tomato and peppers in greenhouse and field trials in 1999. Due to drought conditions during much of the growing season, bacterial spot pressure was low in the tomato trial and there were no significant differences in yield among the treatments. Bacterial spot was moderate-severe in the pepper trial, but none of the biological treatments significantly reduced disease incidence or severity. Experiments to determine the efficacy of these biocontrol agents in transplant production systems are underway.

Systemic Resistance (ISR) to foliar diseases can be induced in several types of vegetable crops grown in composted pine bark-amended planting mixes. However, this effect now is variable depending on the batch of compost used. Only 10 of 80 batches of compost-amended planting mixes tested consistently suppressed bacterial spot of radish and tomato. Over 600 bacterial strains have been recovered from the rhizosphere of radish seedlings grown in the two most ISR-active batches of compost-amended transplant mix. Seventy-one of those strains were able to induce ISR in radish bioassays in one of three tests but none induced ISR consistently. Trichoderma hamatum 382 (T382) has been identified as the most effective fungal biocontrol. The synthetic plant activator Actigard induces systemic acquired resistance consistently when applied to the foliage of tomato plants or radish but it causes a phytotoxic response. Biological treatments applied as amendments to planting mixes for seedlings reduced foliar disease severity in the field when used in combination with conventional chemical treatments in tomatoes and peppers, but not when used alone. Serenade, a commercial Bacillus sp. product, reduced bacterial leaf spot severity in peppers compared to the untreated control. Considerable fine-tuning of the use of these biological control agents will be needed before they can be recommended for commercial production.

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