Angular leaf spot of strawberries
The content of this article 'Angular leaf spot of strawberries' was prepared by Natalia A. Peres and Leandro G. Cordova of the University of Florida and has been revised and republished by FreshFruitPortal.com.
Angular leaf spot (ALS) is a bacterial disease caused by Xanthomonas fragariae, a pathogen highly specific to wild and cultivated strawberry, Fragaria x ananassa.
ALS is an important disease on winter-produced strawberries worldwide and 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 country, including Florida.
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 in California's fruit production fields, it is 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.
Pathogen and symptoms
Xanthomonas fragariae, the causal agent of ALS, is a slow-growing, gram-negative bacterium that produces water-soaked lesions on the lower leaf surfaces (Figure 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 (Figure 2).
A practical way to recognize the disease is to place the leaves against a source of light where the translucent spots can be seen (Figure 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.
This symptom on the calyx is called brown cap, and the lesions are identical to the ones on the leaves (Figure 4). When severe, these calyxes can dry up and make the fruit unmarketable.
Figure 1. Water-soaked lesions of angular leaf spot. Credit: University of Florida, GCREC.
Figure 2. Reddish-brown spots of angular leaf spot. Credit: University of Florida, GCREC.
Figure 3. Translucent spots of angular leaf spot. Credit: University of Florida, GCREC.
Figure 4. Water-soaked lesions of angular leaf spot on the calyx. Credit: University of Florida, GCREC.
Disease development and spread
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.
Limited control options exist when infected transplants are planted in Florida production fields.
Host resistance is not an option to manage ALS since the cultivars grown in Florida are either susceptible or highly susceptible to the disease.
The best way to control ALS is to use pathogen-free transplants.
Since this is not always possible, the use of overhead sprinklers during plant establishment and freeze protection should be minimized to reduce the spread of the disease.
Protective structures, such as high plastic tunnels, are an option to avoid the use of water for freeze protection.
In strawberry production areas where tunnels are used, the disease is usually not a problem. However, production of strawberry in high tunnels is not common in Florida.
These products often help bacteria penetrate through the stomata and thus have shown to 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 sprays preventively and to follow with repeated applications at the low rates.
Our trials have shown that preventive, weekly applications of copper fungicides at 0.3lb of metallic copper per acre were effective in reducing disease symptoms without phytotoxicity to the plants.
However, when disease pressure is low to moderate, the use of copper sprays did not significantly increase yield.
Copper products only increased yield and decreased unmarketable fruit when environmental conditions were highly favorable for infection and dissemination.
Many other products have been tested over the years in the search for an alternative to copper.