Lee Campbell

Pecans are one of the most important horticultural crops in the southeastern United States and are a very important crop for Alabama growers. Each year Alabama ranks from fifth to seventh nationally in pecan production. AAES researchers in 1998 and 1999 used bacteria to fight scab in pecans, offering some hope in the future for reduced cost and more efficient control of this economically damaging condition.

Scab, caused by the fungus Cladosporium caryigenum, is the most serious disease affecting pecans. Pecan scab may damage all parts of the tree during the growing season, but the most devastating effect is on the nuts. Infection on the nuts results in reduced quality and economic loss to the grower. During seasons of normal rainfall, losses in scab-susceptible cultivars that are not protected by fungicides may be as high as 100%. Because of this, control of scab is the most important factor facing pecan growers and is a major economic impact in the production of pecans.

Some of the fungicides commonly used to control scab have been removed or will be removed from the market for use on pecans. As fungicide use has become restricted in recent years and as concerns about the effect of fungicides on the environment and the consumer have developed, researchers are looking at new means for controlling scab. A three-year research project conducted by AAES researchers looked at the use of biological agents as an alternative means of controlling pecan scab.

Greenhouse and field studies were conducted to determine the effect that naturally occurring bacteria had on the control of scab. Using the cultivar ‘Desirable’ initial greenhouse tests were conducted in Auburn to determine which of 30 bacterial isolates gave the best control of scab on pot-grown pecan trees. After the results were analyzed, the two bacteria (Bacillus thuringiensis BP24a and Bacillus pasteurii BT8) that gave the best disease control, were selected for further testing in the field in an established pecan orchard.

Field trials were conducted at the E.V. Smith Research Center in Shorter during the summers of 1998 and 1999. The pecan cultivar ‘Desirable’ was chosen for the test because it is moderately susceptible to scab and is commonly planted in orchards across Alabama and the Southeast. The two bacterial isolates were applied to lower hanging limbs of the trees at a rate of approximately 1 million cells per milliliter of spray volume. Efficacy of the bacteria were compared against the standard fungicide used by growers, which is a combination of Super Tin and Orbit, and an untreated control.

In 1998, during the early part of the summer, which was drier than normal, trees were moderately infected. Near midsummer, scab lesions developed appearing on new foliar growth and developing nutlets. Both biological agents reduced severity of disease on both leaves and nuts; however, biological #1 (B. thuringiensis BP24a) gave the more consistent control on both the leaves and nuts. The disease ratings were similar to that of the standard fungicide treatment and much better than that recorded for the untreated control. Scab severity on the leaves was decreased by an average of 39% on the leaves and by 27% on the nuts as compared with 71% and 23% for the fungicide standard, respectively. Incidence of scab on nuts was also decreased by 58% on the trees sprayed with biological treatment #1 (Tables 1a and 1b).

Table 1a. Disease Severity on Leaves of ‘Desirable’ Pecans, 1998
Treatment	Sample #11		Sample #2		Sample #3
	Mean	% reduction	Mean	% reduction	Mean	% reduction
Untreated Control	15.6	—	22.3	—	23.8	—
Super Tin/ Orbit	3.7	76.3	7.2	67.7	7.2	69.7
Biological #1 Bacillus thuringiensis BP24a	12.6	19.2	13.5	39.5	10.0	57.9
Biological #2 Bacillus pasteurii BT8	11.6	25.6	14.7	34.1	15.7	34.0
1 Samples of 20 leaves per replicate taken on July 9, July 16, and July 23, respectively; means represent the number of lesions of per leaf.

Table 1b. Disease Severity on Nuts of ‘Desirable’ Pecans, 1998
Treatment	With scab lesions1		Disease Ratings2
	Mean	% reduction	Mean	% reduction
Untreated Control	31.0	—	2.2	—
Super Tin/ Orbit	16.3	47.4	1.7	22.7
Biological #1 Bacillus thuringiensis BP24a	13.0	58.1	1.6	27.3
Biological #2 Bacillus pasteurii BT8	27.5	11.3	2.0	9.1
1Means represent the total percentage of nuts with scab lesions.
2Ratings of disease on nuts harvested August 14; mean represents the average of all nuts rated. Rating scale of 1 to 5 where 1 = clean, no lesions; 2 = trace to 25%; 3 = 26 to 50%; 4 = 51 to 75%; and 5 = >75% of surface covered by lesions.

Weather conditions during 1999 were more conducive for disease development. Scab lesions appeared on the leaves and nuts much earlier in the 1999 growing season compared to 1998. Heavy scab pressure resulted in poorer disease control than was observed during the previous year’s field trial. Disease ratings on the leaves were comparable to the 1998 results, but disease on the nuts was more severe than in 1998. Biological #1 again gave results comparable to those obtained in 1998 but biological #2 (Bacillus pasteurii BT8) gave slightly better control of scab on the leaves (28% reduction compared to 30% reduction). This was compared to a reduction of 68% for the standard (Table 2). Due to heavy infection by the scab fungus, neither biological agent reduced severity of disease on the nuts when compared to the untreated control.

Table 2. Disease Severity on Leaves and Nuts on ‘Desirable’ Pecans, 1999
Treatment	Leaves Disease Ratings						Nuts Disease Ratings2
	Sample #1 1		Sample #2		Sample #3
	Mean	% Reduction	Mean	% Reduction	Mean	% Reduction	Mean	% Reduction
Untreated Control	35.4	—	60.7	—	97.5	—	4.3	—
Super Tin/Orbit	9.7	72.5	18.9	68.9	36.6	62.4	2.2	48.8
Biological #1 B. thuringiensis BP24a	23.5	33.6	40.8	32.8	79.5	18.5	4.2	2.3
Biological #2 B. pasteurii BT8	20.8	41.2	50.5	16.8	65.0	33.3	4.2	2.3
1 Samples of 20 leaves per replicate taken on June 3, June 10, and June 18, respectively; means represent the number of lesions of per leaf.
2Ratings of disease on nuts harvested August 14; mean represents the average of all nuts rated. Rating scale of 1 to 5 where 1 = clean, no lesions; 2 = trace to 25%; 3 = 26 to 50%; 4 = 51 to 75%; and 5 = >75% of surface covered by lesions.

The results of this study show the potential for biological organisms as an alternative to traditional fungicides for control of scab on pecans. Further work is needed to determine the effectiveness of biological agents over an extended period of time under various environmental conditions, the proper means of application, the maximum effectiveness of biological agents against pecan scab and other pecan diseases, and whether fungicide use can be reduced.