Roberto Pereira & Bonnie Ownley

A field experiment was conducted to compare the effectiveness of microbial control agents and chemical pesticides when used on field tomatoes. Treatments included all combinations of chemical, biological or no control of either diseases or insect pests. Irrigation problems prevented uniform development of the crop in the field, and caused one experimental block to have significantly lower yield than others. Low yields may have masked differences among treatments, among which no significant differences were observed in yields. Treatments receiving biological and chemical control of diseases were not significantly different for southern blight, but were different for early blight incidence.

Commercial and experimental biological control agents are available for use against several insect pests and pathogens of tomatoes. However, their commercial use suffers from a lack of understanding about their efficiency, and the possibility of their integration with other forms of insect and disease control. A field experiment was conducted: a) to evaluate the effects of microbial agents for control of insect pests and plant pathogens and insect pests in tomato production in East Tennessee; b) to compare the effectiveness of these microbial control agents with other strategies (traditional pesticides); and c) to compare the effects of disease and pest control strategies on yield of field tomatoes.

Tomato seedlings (cv. Mountain Pride) were produced in the greenhouse from three types of seeds: a) seeds with no treatment (Control); b) seeds coated with Beauveria bassiana conidia in a 2% methyl cellulose suspension (Biological); or c) seeds commercially treated with the chemical fungicide Thiran (Chemical). Seedlings were transplanted to the field on May 8, 2000. Drip irrigation was applied, and management practices followed established standard procedures, except insect and disease control practices, which were done according to treatments as described on Table 1.

Each experimental plot consisted of three rows of seven tomato plants (21 plants per treatment plot). Rows within plots were separated by 6 ft and plants within a row were separated by 18 in. Four blocks (replicates) of treatment plots were planted; each block consisted of 3 rows of plants separated from the adjacent blocks by 10 ft. Plots within a block were separated by 5 ft. Treatments were arranged in a randomized complete block design. Only the seven plants in the central row in each plot were used for disease incidence evaluations and yield data. Other plants served as borders and allowed observation and collection of insects and plants parts without disturbance of the central plants.

During the growing season, scouting for insects was done at weekly intervals to determine the need for insect control measures. At the same time, plants were inspected for possible signs of disease incidence to determine the need for disease control. Assessments of disease incidence/severity were done by counting plants with disease symptoms (for southern blight caused by Sclerotium rolfsii), or estimating percent leaf area with disease (for early blight caused by Alternaria solani).

Scouting indicated the need for a single application of insect control measures. The biological control plots received a mixture of the fungus B. bassiana (Mycotrol at 1 qt/acre), for control of aphids and other sucking insects, and Bacillus thuringiensis (Agree at 2 lb/acre), for control of caterpillars. The chemical control plots received the recommended rate of the insecticide esfenvalerate (Asana at 9.6 fl oz/acre). Two applications of disease control products were necessary. One application was made for control of S. rolfsii, and one for control of A. solani. For control of S. rolfsii, the biological control plots received one application of the fungus Gliocladium virens (Soilgard, at 0.5 oz at the base of each plant), and the chemical control plots received PCNB (Terrachlor F at 4.5 pt / 100 gal water; 0.5 pt per plant). For control of A. solani, sodium bicarbonate (1 tbsp/gal water; plants sprayed until runoff) was used in the biological control plots, and mancozeb (Dithane DF at 3 lb /Acre) was used in chemical control plots.

Table 1. Scheme of disease and insect control treatments for field test

As tomatoes matured, they were harvested between July 17 and Aug. 1, when all remaining fruits were collected. Fruits were counted, weighed and graded according to size and quality. The yield (number of tomatoes, number of #1 + #2 tomatoes, weight of all tomatoes, weight of #1 + #2 tomatoes, and mean weight per fruit) and the disease incidence data were analyzed by ANOVA using the SuperAnova statistical package.

Irrigation problems prevented uniform development of the crop in the field. Because fertilizer was added to the irrigation water, addition of plant nutrients to the field was also not uniform. Lack of water may also have affected the incidence of diseases.

Block 3 suffered the most yield loss due to the lack of irrigation (Table 2). This block had lower total yield (14.8 tomatoes, for a total of 2.1 kg fruits/plot), and lower yield of #1 and #2 tomatoes (6.7 #1+#2 tomatoes, for a total of 1.4 kg fruits/plot) than the average of the other 3 blocks (23.0 tomatoes, for a total of 4.0 kg of fruits/plot, 14.6 #1+#2 tomatoes, for a total of 3.1 kg fruits/plot). The mean weight of tomatoes from block 3 was 0.153 kg per tomato, compared with 0.173 kg/tomato in other blocks.

Table 2. Tomato yield in different blocks which included nine treatments combining disease and insect control strategies as defined in Table 1.

Means followed by the same letter within a column are not significantly different (Fisher’s Protected Least Significant Difference at 0.05 confidence level).

No significant differences were observed among the treatments in any of the yield variables measured (Tables 3 and 4). Total yields averaged 20.8 tomatoes (#1 - #5), or 3.5 kg per plot, equivalent to 14,373 tomatoes or 5,321 lb. per acre. The high grade tomato yield averaged 12.7 tomatoes (#1 + #2), or 2.7 kg per plot, equivalent to 8,776 tomatoes or 4,105 lb. per acre. These yields are very low in relation to the yield potential of the tomato cultivar used in the experiment, which can produce more than 20 tons of tomatoes per acre. Besides factors related to the irrigation problems, other factors such as soil, climate, bird and other vertebrate damage to fruits,

disease incidence, insect damage, and others may account for the low yield. The mean weight of tomato was 0.17 kg per fruit, equivalent to 6.0 ounces per fruit. Therefore, size of the harvested fruits were within the range expected for the Mountain Pride cultivar.

Table 3. Total tomato yield (number of fruits and weight) of nine treatments combining disease and insect control strategies.

Disease incidence was low during the 2000 tomato field season. The highest percent incidence of southern blight was 8.3 ± 3.71 % in the plots receiving no treatment for control of diseases. This value was significantly different from the incidence of S. rolfsii in either the plots treated with biological control agents (1.2 ± 1.2 %) or those receiving chemical disease control (0%). The incidence of southern blight was not significantly different between the biological and the chemical control plots.

Table 4. High grade tomato yield (number of #1 and #2 fruits and weight) of nine

treatments combining disease and insect control strategies.

No significant differences were observed among the treatments.

The incidence of A. solani (early blight) was highest in the plots receiving biological control of plant diseases (17.0 ± 2.18 % of foliage affected by the disease), and lowest on the chemical control plots (9.6 ± 1.89 %). These two treatments were not significantly different from the control that had 11.7 ± 2.16 % of affected foliage, both the chemical and the biological control treatments were significantly different from each other. These results demonstrate that the biological control measures used to control plant diseases were not effective against the early blight fungus, although adequate control of southern blight was obtained. However, the low disease incidence in the field was not sufficient to cause differences in tomato yields. The low yields obtained in this experiment due to other factors may have masked any effect that diseases and insect pests had on tomato yield.