• Topics: Strawberry Diseases | Plant Pathology | Gulf Coast REC | Cantliffe, Daniel J | Price, James F | Rondon, Silvia Ivette | Peres, Natalia A | Xanthomonas spp.

Angular Leaf Spot of Strawberries¹

Angular leaf spot (ALS) is a bacterial disease caused by Xanthomonas fragariae, a pathogen highly specific to both the wild and the cultivated strawberry, Fragaria x ananassa.

ALS is an important disease on winter-produced strawberries worldwide, and it is considered a quarantine disease by the European Union. The disease was reported first in the United States in Minnesota in 1960; since then, it has been found in almost all cultivated strawberry areas in the United States. In California, the largest U.S. producer of fresh strawberries, ALS is a minor disease that occurs especially during rainy weather or when overhead sprinkler irrigation is used. Although the disease is considered minor for fruit production fields in California, it is considered a problem for nurseries that export plants to Europe and other countries with quarantine restrictions.

In Florida, ALS is the only strawberry disease caused by a bacterium. The disease can be especially problematic during seasons when overhead irrigation is used many times for freeze protection.

Pathogen and Symptoms

X. fragariae, the causal agent of ALS, is a slow-growing, Gram-negative bacterium that produces water-soaked lesions on the lower leaf surfaces (Fig. 1). The bacteria enter the leaf through the stomata (tiny spores that are most abundant on the lower surface of the leaf). Lesions begin as small and irregular spots on the undersurface of the leaflets. When moisture is high on the leaves, lesions ooze sticky droplets of bacteria. As the disease develops, lesions enlarge and coalesce to form reddish-brown spots, which later become necrotic (Fig. 2).

A practical way to recognize the disease is to place the leaves against a source of background light where the translucent spots can be seen (Fig. 3). The tissue with older damage eventually dies and dries up, giving leaves a ragged appearance.

During severe epidemics, the pathogen also can cause lesions on the calyx of fruit that are identical to foliar lesions (Fig. 4). When severe, these calyxes can dry up and make the fruit unmarketable.

Figure 1. 

Water-soaked lesions of angular leaf spot. (Credits: University of Florida, GCREC)

[Click thumbnail to enlarge.]

Figure 2. 

Reddish-brown spots of angular leaf spot. (Credits: University of Florida, GCREC)

[Click thumbnail to enlarge.]

Figure 3. 

Translucent spots of angular leaf spot. (Credits: University of Florida, GCREC)

[Click thumbnail to enlarge.]

Figure 4. 

Water-soaked lesions of angular leaf spot on the calyx. (Credits: University of Florida, GCREC)

[Click thumbnail to enlarge.]

Disease Development and Spread

Little is known regarding the epidemiology of ALS; however, the development of the disease seems to be favored by warm days (68°F) and cold nights (36–39°F).

The primary source of inoculum in a new field is contaminated transplants. Secondary inoculum comes from bacteria that exude from lesions under high moisture conditions. Bacteria can survive on dry infested leaves and tissue buried in the soil for up to one year.

The pathogen can be spread easily by harvesting operations when wet and cool conditions favor the production of bacterial exudate. The pathogen also can be dispersed by rain and overhead sprinkler irrigation.

Control

The best way to control ALS is to use pathogen-free transplants. Since this is not always possible, growers should avoid harvesting and moving equipment through infested fields when the plants are wet. Minimizing the use of overhead sprinklers during plant establishment and for freeze protection also reduces the spread of the disease. The use of surfactant-type spray adjuvants should also be avoided when ALS is a threat since these products often help bacteria penetrate through the stomata and may enhance disease development.

Copper-based products can provide effective control of the disease in some instances, but low rates of copper should be used since phytotoxicity (reddening of older leaves, slow plant growth, and yield decrease) has been documented with repeated sprays. A number of copper products are labeled for ALS control on strawberry, such as copper hydroxide, copper oxychloride, basic copper sulfate, cuprous oxide, and various other copper compounds. All of these active ingredients suppress ALS, but it is important to apply the correct amount. Trial results have shown that preventive, weekly applications of copper fungicides at 0.3 lb of metallic copper per acre were effective in reducing disease symptoms without causing phytotoxicity on the plants. However, trial results have also shown that when disease pressure is low to moderate, the use of copper sprays did not significantly increase yield. Copper products can increase yield and decrease the possibility of fruit rejection only when environmental conditions are highly favorable for infection and spread.

Many other products have been tested over the years in the search for an alternative to copper. Actigard^®, a plant-resistant activator manufactured by Syngenta, has been shown to suppress ALS. Actigard^® is used to control bacterial spot disease on tomatoes in Florida, but it is not currently approved for use on strawberry. However, registration materials have been submitted, and this product may be available for use during the 2011-12 strawberry season.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, Dean.

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