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Alternaria brown spot, caused by Alternaria alternata, affects Minneola tangelos, Dancy tangerines, Murcott tangor and less frequently, Orlando tangelos, Novas, Lees and Sunburst tangerines. In rare cases it may also infect grapefruit.

This disease causes serious defoliation, fruit drop and fruit blemishes and is a limiting factor in the production of these cultivars in humid areas. Even in semi-arid production areas, blemishes on the peel can significantly reduce marketability of the fruit (Timmer et al., 2000).

In humid climates such as that of Florida, fungal diseases such as Alternaria brown spot are difficult to control. Protectant fungicides such as copper products are still the mainstay of control programs for this disease (Timmer, 2002). However, copper accumulates in soils and can cause phytotoxicity (Alva and Graham, 1991); therefore its use needs to be limited in some plantings and in hot, dry weather.

The economics of citrus production preclude frequent application of fungicides. Currently under ideal climatic conditions, growers may apply 9-10 sprays per season to control this disease. These costs are prohibitive and can be as much as $500 per acre (N.A. Smith, personal communication). Strobilurins have recently been registered for citrus and provide good control of Alternaria brown spot (Timmer, 2004). However these fungicides must be managed carefully to avoid future problems with the development of resistance.

Systemic induced resistance has been investigated extensively for the control of many plant diseases. A wide range of compounds such as benzothiadiazoles, salicylic acid, harpin protein, fatty acids and oligosaccharides are known to be effective inducers of plant resistance to disease. Products that induce resistance may be useful in the control of foliar fungal diseases of citrus.

It is difficult to control these diseases on rapidly expanding leaves and fruit with protectant fungicides. Induced resistance could therefore provide systemic protection against infection to substitute for, or supplement control by standard fungicides.

Products such as Messenger (Harpin protein), ProPhyt (potassium phosphite), Nutriphite (phosphorous acid), Aliette 80 WP (fosetyl- Al), KeyPlex 350DP and 445 DP (α-keto acids), Oxycom Respond (hydrogen peroxide), ReZist, Goemar H11 (laminarin), Serenade (Bacillus subtilis), and Actigard (acibenzolar-s-methyl) have been tested against Alternaria brown spot in the laboratory and greenhouse (Agostini et al., 2003).

In greenhouse studies, most of these products reduced disease severity compared with the untreated control, but were all less effective than the standard Abound fungicide. ProPhyt and Rezist have proven effective when used in early sprays for brown spot control (Bhatia and Timmer, 2003).

The purpose of the current study was to assess the efficacy of Agromos™ and ISR 2000™ to control Alternaria brown spot under field conditions.


Materials and methods

Fungicides were evaluated in an 8-year-old grove of Murcott tangors near Bereah, Florida in 2003. Each treatment was applied to five two-tree plots arranged in a randomized complete block design. Unsprayed guard trees were located between the plots. All sprays were applied with a handgun at 200 psi. The desired rate per acre for mature groves was added to 125 gallons of water and the trees sprayed to runoff using approximately two gallons per tree.

The first application was made on March 20 when the spring flush was up to 1/4 expanded to reduce inoculum production on old foliage and prevent infection of new growth. A second application was made on April 11 after petal fall to reduce spore production on leaves and protect young fruit from infection. The third application was made on May 13 when fruit was about 1/4-1/2 inch in diameter, approximately 4-5 weeks after petal fall. A fourth application was made on June 19 about 8-10 weeks after petal fall to protect fruit from late Alternaria infection.

Rainfall was very high especially during May and June. Total precipitation between the first and second applications was 6.2 inches, between the second and third it was 8.5 inches and between the third and the fourth it was 20.7 inches. In the period following the last application until July 15, a total of 30.4 inches fell. Thus conditions were highly favorable for disease development this season. However, Murcott is not the most highly susceptible variety and inoculum levels were generally low at the beginning of the season. Despite favorable conditions, damage from brown spot was only moderate on unsprayed controls.

In November, 50 fruit per tree were rated for severity of Alternaria brown spot on a scale of 0 = no disease; 1 = mild disease, suitable for the fresh market; 2-5 = fruit useful only for processing, increasing severity. The percentage of marketable fruit, those rated as 0 or 1, was calculated for each treatment. Data were subjected to analysis of variance and means separated by the Waller Duncan k-ratio t test, P<0.05.


Results and discussion

All treatments reduced disease severity significantly compared to the unsprayed control. Serenade was least effective and all other treatments were significantly more effective than this product.

Some minor phytotoxicity occurred in the treatments where copper fungicides were applied at a rate of 4.3 lb/acre. The phytotoxicity in combination with ISR 2000™ may have been as a result of too high a rate of copper applied to tender leaf tissue. ISR 2000™ contains an ingredient called Crop-Set™, that enhances vegetative growth of citrus (Medina, 2003).

Two applications of either Agromos™ or ISR 2000™ followed by two applications of copper (Kocide 2000) were as effective as four applications of standard copper (Table 1). Four applications of copper resulted in 93% marketable fruit while the combination of copper and Agromos™ or ISR 2000™ resulted in 91% and 92% marketable fruit, respectively. The untreated controls produced 62% marketable fruit.

Agromos™ and ISR 2000™ were not tested alone, but previous results with compounds that induce systemic resistance indicated that they do not achieve the same level of control as standard fungicides (Agostini et al., 2003). This may be true for Agromos™ and ISR 2000™ as well, but these products are more user-friendly than products such as Serenade.

The level of control achieved by these products may not be as high, but organic growers could benefit from them especially if the use of copper fungicides is restricted. Although resistance-inducing compounds such as Agromos™ and ISR 2000™ may not be substitutes for current commercial fungicides, they can be used with great effect when applied earlier to protect rapidly expanding leaf tissue and avoid inoculum buildup. Furthermore, they reduce the number of copper sprays, which in turn reduces the amount of copper reaching the soil.


Table 1. Effect of Agromos™ and ISR 2000™ in combination with Kocide 2000 on the control of Alternaria brown spot in Murcotts.


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References

Agostini, J.P., P.M. Bushong and L.W. Timmer. 2003. Greenhouse evaluation of products that induce host resistance for control of scab, melanose and Alternaria brown spot of citrus. Plant Dis. 87:69-74.

Alva, A.K. and J.H. Graham. 1991. The role of copper in citriculture. Adv. Agron. 1:145-170.

Bhatia, A. and L.W. Timmer. 2003. Evaluation of products for the control of Alternaria brown spot on Minneola tangelos, 2002. Fungicide and Nematicide Tests (online) Report 58: MO06.DOI.10.1094/FN58. The American Phytopathological Society, St. Paul, MN.

Medina, C.L. 2003. Comparative effects of gibberellic acid, Crop-Set™ and potassium nitrate on vegetative development, fruit set and quality of orange fruits, Citrus sinensis (L) Osbeck. In: Proceedings 19th Annual Alltech Symposium, Lexington, KY. (T.P Lyons and K.A. Jacques, eds.) Nottingham Univ. Press. Pages 407-414.

Timmer, L.W. 2004. Florida Citrus Pest Management Guide. Univ. Fla. Inst. Food Agric. Sci. Publ. No. SP-43.

Timmer, L.W., Z. Solel and M. Orozco-Santos. 2000. Alternaria brown spot of mandarins. In: Compendium of Citrus Diseases. (L.W. Timmer, S.M. Garnsey and J.H. Graham, eds.) APS Press, St. Paul, MN. Pages 19-20.

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