tomato growth

HIGHLIGHTS.Online • • • Summer 2001

Research from the Alabama Agricultural Experiment Station of Auburn University

Can Beneficial Bacteria from Auburn Boost Tomato Growth and Yield?

Commercial production of tomato in the United States is dependent on the use of transplants, which has generated a thriving transplant plug industry. It is essential to have an adequate supply of robust and disease-free transplants, and AAES researchers are finding that beneficial bacteria can be an effective and ecologically benign way to assure this supply.

Using transplant plugs instead of planting from seed is more popular because transplants offer a more uniform plant size, are easier to plant, and transplants often produce increased fruit production in commercial greenhouse facilities. However, after transplanting in the field, young tomato plants are immediately exposed to a large number of fungal, bacterial, nematode, and viral pathogens that cause diverse diseases. Major tomato diseases include those that attack the root system (Fusarium wilt, Verticillium wilt, bacterial wilt, nematodes, crown and root rot), aboveground stems and foliage (early blight, septoria leaf spot, bacterial canker, late blight), and fruit (bacterial spot, bacterial speck, anthracnose). Thus, successful tomato production is dependent upon disease management programs throughout the life of the tomato plant.

Current plant disease control techniques are largely based on several applications of chemical fungicides. However, the availability of chemical pesticides is declining and therefore the need for alternative control strategies, including those that would allow growers to use an organic label for fruit, is growing. Work has been on-going at Auburn University on the use of beneficial bacteria (called PGPR for “plant-growth-promoting rhizobacteria”) as components in a biologically based integrated pest management strategy.

PGPR increase plant growth of a number of agronomically important crops, and some PGPR strains activate plant defenses, a phenomenon known as “induced systemic resistance” (ISR). ISR using PGPR has been noted against multiple pathogens, including fungi, bacteria, viruses, and, in a very few cases, nematodes. An alternative approach for control of nematodes is the use of specific organic amendments, such as chitin and chitosan, which control nematodes when mixed into field soils. AAES scientists have been investigating the combination of PGPRs with chitosan amendments in peat-based plant growth media for production of vegetable transplants. The combination of PGPR with chitosan is designed to enhance seedling growth and develop disease-suppressive transplant plugs.

The study’s goals were to increase the rate of seedling growth of several vegetable transplant plugs and thereby decrease the time required to produce transplants in commercial greenhouse prior to transplanting into fields and to develop disease suppressive transplant plugs, which are protected for a time from multiple diseases.

The effects of several mixtures of PGPR and chitosan on plant growth-promotion and ISR against various foliar pathogens were evaluated under greenhouse conditions. Each biological preparation contained industrially formulated spores of two PGPR strains and flaked chitosan. Field experiments were conducted during 1999 at Sand Mountain Research and Extension Center in Crossville to evaluate several of the biological preparations as potting mix amendments at seeding application for control of root-knot nematode and bacterial spot of tomato. Further, the effects of biological treatments on growth and yield of tomato were evaluated in a field naturally infested with root-knot nematode and foliar pathogens. Seedling growth parameters were measured at 0, 30, and 60 days after transplanting. Fruit was harvested and weighed at several times during the growing season.

The study showed that most of the biological preparations, particularly BioYield™, significantly increased seedling growth of tomatoe, tobacco, cucumber, and pepper (measured by height and caliper) compared to a nontreated control (see figures 1 [17K]and 2 [17K]). Most of the prepartions provided significant levels of disease suppression for all diseases evaluated (data not shown here).

Mixtures of two PGPR strains plus a formulation carrier chitosan, particularly BioYield™, exhibited more consistent and significant levels of growth promotion on tomato seedlings. The transplant mix delivery system for biological preparations has demonstrated potential to enhance transplant vigor, provide protection against root-knot nematodes and other foliar diseases in the field, and enhance yield (see figure 3 [19K]). These results indicate a synergy in plant growth-promotion and induced resistance by the combination of chitosan and the mixtures of two bacterial strains. Gustafson, LLC, has developed one of the biological preparations designed at Auburn University into a commercial product under the trade name of BioYield™. This research has been conducted in collaboration with the Sand Mountain Research and Extension Center. Studies are in progress to test BioYield^™ as a component for integrated pest management systems in many other vegetable transplant systems and also in row crops in Alabama.

Reddy is Associate Research Professor, Ryu is Graduate Research Assistant, Rodríguez-Kábana is Distinguished University Professor, and Kloepper is a Professor of Entomology and Plant Pathology; Dawkins is Superintendent of the Sand Mountain Research and Extension Center.