Lee Campbell, Bill Moss, Jan Byrne, Alexei Dianese, and Mark Wilson

Bacterial spot of tomato, caused by Xanthomonas axonopodis pv. vesicatoria, is a severe problem affecting fresh-market tomato production in Alabama and has been rated by growers as one of the top three disease problems on tomatoes in the southeastern United States. Recent AAES studies indicate that beneficial bacteria may provide effective and environmentally benign control methods for this disease.

Bacterial spot affects foliage, blossoms, and stems, but the most serious damage occurs to the fruit that may be rendered unmarketable. Currently, copper bactericides in combination with EDBC (ethylene bisdithiocarbamate) fungicides are the primary means of control of this disease; however, copper resistance has developed in the pathogen population and restrictions are being placed on the use of EDBC fungicides in tomato production due to concerns of carcinogenicity.

AAES researchers, in collaboration with researchers in each of the major tomato-growing regions of North America, are currently exploring alternatives for the control of bacterial spot using naturally occurring beneficial bacteria.

(Top) Xanthomonas axonopodis pv. vesicatoria--pathogen that is causal agent of bacterial spot. (Bottom) Pseudomonas syringae Cit7--biological control agent that gave most consistent results.

Naturally occurring beneficial bacteria were isolated from tomato leaves and other plant species and were screened under greenhouse conditions to assess their ability to reduce the severity of X. axonopodis pv. vesicatoria infection on tomato foliage. After numerous screenings, the five bacterial isolates that consistently provided the highest level of disease reduction were selected for further testing in field trials in 1996. In 1997, Pseudomonas putida B56 and P. syringae Cit7, which had given the most consistent results in trials in Alabama and Florida, were further tested under field conditions in Alabama, Florida, Ohio, North Carolina, and Ontario, Canada. In Alabama, field trials were conducted at the E.V. Smith Research Center in Shorter during 1996 and 1997 using tomato cultivar Agriset 761. In both years, warm temperatures and moderate rainfall were conducive to the development of bacterial spot and in 1997 disease incidence increased progressively throughout the season.

In 1996, bacteria were applied to the foliage at weekly intervals at a concentration of 100 million cells per milliliter. This biological treatment was compared with the standard copper and maneb chemical treatment (Kocide and Manex). Foliar disease severity ratings were made on three separate occasions. Disease severity on leaves was quantified as the average number of lesions per leaflet derived from a sample of 20 leaflets per replicate row. Fruit disease incidence (percentage) was determined from a complete harvest on November 1.

In 1997, a follow-up test was conducted at two separate sites. The biological and chemical agents were applied weekly to the foliage at the same concentrations used in 1996. In field 1, foliar disease severity ratings were made on five separate occasions and were based on a sample of 20 leaflets per replicate row. In field 2, foliar disease severity ratings were made on four separate occasions. Disease incidence on fruit was too low to rate in field 2, but a complete harvest still was made from this field to determine the effect that the biological organisms had on total fruit number (i.e. yield).

In 1996, P. putida B56 and P. syringae Cit7 gave the most consistent reductions in disease severity. The average number of lesions per leaflet was reduced by an average of 21% and 39%, respectively, compared with the chemical treatment, which reduced the average number of lesions per leaflet by 54.9% (Table 1). P. putida B56 and P. syringae Cit7 also reduced incidence of infected fruit by 35% and 34%, respectively, compared to 17.9% with the chemical treatment (Kocide + Manex) (Table 1). This demonstrated the ability of the biological agents to not only reduce foliar disease severity but also to increase the yield of marketable fruit.

Table 1. Results from 1996 Field Trials

Treatments	Reduction in foliar disease severity, pct.1			Reduction in fruit disease incidence, pct.2
	Oct.17	Oct.24	Nov.1
Nontreated control	---	---	---	---
Kocide® + Manex®	38.6	58.9	67.4	17.9
Pseudomonas flourescens A506	0.0	23.5	17.4	0.0
Ervinia herbicola B2	0.0	8.7	16.3	0.0
Pseudomonas mendocina B41	0.0	6.9	6.5	0.0
Pseudomonas putida B56	30.7	30.9	1.1	35.0
Pseudomonas syringae Cit7	36.4	41.2	39.1	34.3
1Represents samples of 20 leaflets per replicate row. 2Represents the total number of fruit harvested and the reduction in number of infected fruit.

In 1997 in field 1, P. putida B56 and P. syringae Cit7 reduced the average number of lesions per leaflet by an average of 15% and 30%, respectively, compared to 40% with the chemical treatment (Table 2). In field 2, P. putida B56 and P. syringae Cit7 reduced the average number of lesions per leaflet by 39% and 42%, respectively, compared to 48% with the chemical treatment, and increased the number of marketable fruit by 20% and 25% over the nontreated control, compared to 61% with the chemical treatment (Table 2).

Table 2. Results from 1997 Field Trials

Treatments	Reduction in foliar disease severity, pct.1									Increase in fruit number over control, pct. 2
	Test 1					Test 2
	Sept. 19	Sept. 29	Oct. 3	Oct. 10	Oct. 20	Sept. 29	Oct. 6	Oct. 13	Oct. 23
Nontreated control	---	---	---	---	---	---	---	---	---	---
Kocide®+Manex®	66.2	32.5	53.2	33.8	16.8	55.8	30.6	56.3	51.7	61.6
P. putida B56	31.9	26.0	4.3	10.7	21.0	44.2	30.6	49.2	32.5	19.4
P. syringae Cit7	48.8	34.1	27.5	20.5	18.7	46.1	22.4	57.7	42.7	24.8
1Represents samples of 20 leaflets per replicate row. 2Represents the total number of fruit harvested.

Results from Florida, Ohio, North Carolina, and Canada also showed that P. putida B56 and/or P. syringae Cit7 were effective in reducing foliar severity of bacterial spot. These combined results demonstrate the potential of biological agents as a way of controlling foliar diseases of tomato. Further, the foliar bacteria may be used in an integrated system with bacteria that are applied to the root system to induce systemic resistance in plants or with CGA-245704 (Novartis Crop Protection, Inc.), a chemical that induces systemic resistance within the plant. Both of these integrated approaches are being examined by AAES researchers in 1998 field trials.