Foliar Diseases

There are three major leaf diseases of strawberries in the Midwest. They are leaf spot, leaf scorch, and leaf blight (Figure 1). All three diseases can occur singly or together on the same plant or even on the same leaf. All three are caused by fungi. Under favorable environmental conditions, these three diseases can cause serious reductions in strawberry yields. They damage the strawberry plant by causing premature leaf death, reduction in fruit quality, a general weakening of the plant, and (in some situations) plant death. In order to maximize strawberry production, these leaf diseases must be recognized and controlled. Fortunately, several varieties have good resistance to leaf spot and leaf scorch (Table 1).

 
(Figure 1) Leaf spot and leaf scorch usually appear first in early to mid-spring. Leaf blight is more common during the summer and early fall.

Leaf Spot

Symptoms

Leaf spot is caused by the fungus Mycosphaerella fragariae. The leaf spot fungus can infect leaves, fruit, petioles, runners, fruit stalks, and berry caps or calyxes. The most obvious symptoms of the disease are small, round spots. These spots develop on the upper surface of the leaf and at first are dark purple to reddish-purple (Figure 2). They range in size from 1/8 to 1/4 inch in diameter. With time, the centers of the spots become tan or gray and eventually almost white; while their margins remain dark purple. Later in the season, tan or bluish areas form on the underleaf surface. Symptoms on other plant parts, except fruit, are almost identical to those on the upper leaf surface. On fruits, superficial black spots may form during moist weather (Figure 3). The spots form on ripe berries and around groups of seeds. They are about 1/4 inch in diameter, and usually there are only one or two spots per fruit. However, some fruits may be more severely infected.

 
(Figure 2) Strawberry leaf spot symptoms on leaflet.

(Figure 3) Black seed disease on strawberry fruit. This disease is caused by the same fungus that causes strawberry leaf spot.

Disease Development

This fungus can produce two types of spores that infect newly-emerging leaves in spring. First, older infected leaves that remain alive during winter may give rise to conidia (spores) that are spread to new foliage by splashing water or by handling infected plants. Another type of spore (ascospore) is produced in speck-sized black perithecia, which form at the edges of the leaf spots during autumn. In the spring, these ascospores are forcibly ejected from perithecia and are carried by wind or water to new leaf tissue.

Infection by both types of spores occurs through the underleaf surface. Temperatures between 65 and 75 F are optimum for infection and disease development. Infections may occur throughout the growing season, except during hot, dry weather. Young, expanding leaves are the most susceptible to infection.

Leaf Scorch

Symptoms

Leaf scorch is caused by the fungus Diplocarpon earliana. The leaf scorch fungus can infect leaves, petioles, runners, fruit stalks and caps of strawberry plants. Leaf scorch symptoms are very similar to the early stages of leaf spot. Round to angular or irregular dark-purple spots up to 1/4 inch in diameter are scattered over the upper leaf surface (Figure 4). As spots enlarge, they resemble small drops of tar. This tar-like appearance is caused by the formation of large numbers of minute, black, fungal fruiting bodies (acervuli). The centers of the spots remain dark purple. This distinguishes the disease from leaf spot where the center turns white. If many infections occur on the same leaf, the entire leaf becomes reddish or light purple. Severely infected leaves dry up and appear scorched. Similar, but elongated, spots may appear on other affected plant parts. Lesions may girdle fruit stalks causing flowers and young fruit to die. Infections on green berries are rare, appearing as red-to-brown discolorations or a flecking on the fruit surface. The leaf scorch fungus can infect strawberry leaves at all stages of development.

(Figure 4) Leaf scorch on strawberry. First symptoms are individual red spots.

Disease Development

The fungus overwinters on infected leaves that survive the winter. In the spring, conidia are produced on both leaf surfaces in speck-sized, black acervuli. The fungus also produces ascospores in the early spring within disk-shaped apothecia (fungal fruiting structures) that appear as black dots in old lesions on the lower surface of diseased leaves that died during winter. In the presence of moisture, ascospores germinate within 24 hours and infect the plant through the lower leaf surface. After symptom development, conidia are produced on the leaf spots in large numbers throughout the growing season. Therefore, repeated infections occur whenever weather conditions are favorable. Conidia are spread mainly by splashing water.

Symptoms

Leaf blight is caused by the fungus Phomopsis obscurans. Leaf blight is found most commonly on plants after harvest. The disease is distinctively different from both leaf spot and leaf scorch. The enlarging leaf spots of this disease are round to elliptical or angular and a quarter of an inch to an inch in diameter (Figure 5). Spots are initially reddish-purple. Later, they develop a darker brown or reddish-brown center surrounded by a light-brown area with a purple border. Similar spots may sometimes develop on the fruit caps. Usually, only one to six lesions develop on a leaflet. Often the infected area becomes V-shaped with the widest part of the "V" at the leaf margin. New lesions appear throughout the summer and fall if weather conditions are favorable. Older leaves become blighted and may die in large numbers. This disease is usually more destructive on slow growing or weak plants. The same fungus can cause an enlarging, soft, pale-pink rot at the stem end of the fruit. Information on resistance to leaf blight in currently used varieties is limited. If growers encounter a high level of disease on certain varieties, these varieties should be avoided.

(Figure 5) Phomopsis leaf blight on strawberry.

Disease Development

This fungus produces spores (conidia) in speck-sized, black pycnidia (fungal fruiting bodies) embedded in the centers of older leaf lesions. Conidia ooze out of pycnidia during damp weather when temperatures are high. Conidia are splashed to new leaf tissue where they germinate in the presence of free water to initiate new infections on leaves and fruit. The fungus overwinters on either infected leaves that survive the winter or in dead tissue on old infected leaves.

Powdery mildew is caused by the fungus Sphaerotheca macularis. Generally the disease is not a serious problem in the Midwest; however, under the proper environmental conditions and on highly susceptible varieties, the disease can become serious. Disease resistance is available in several varieties. Growers are encouraged to avoid highly susceptible varieties.

Symptoms

Foliage symptoms usually are the most obvious. An upward curling of leaf edges usually is the first symptom seen. Dry, purplish or brownish patches develop on the lower surface of infected leaves and reddish discoloration may develop on the upper surface (Figure 6). Patches of white, powdery fungus mycelium may appear on the undersides of leaves as the disease progresses (Figure 7).

(Figure 6 )Reddish-purple discoloration of leaf often associated with powdery mildew infection

( Figure 7) Patches of white fungus growth on strawberry infected with powdery mildew.

Disease Development

The fungus that causes strawberry powdery mildew infects only wild and cultivated strawberries. This pathogen can not survive in the absence of living host tissue. It apparently overwinters in infected leaves. Spores are carried by wind to infect new growth in the spring. Development and spread of powdery mildew is favored by moderate to high humidity and temperatures of about 60 to 80 F (15 to 27 C). Unlike most other fungi that cause plant disease, powdery mildew does not require free water for spores to germinate and infect. In dry years, when most other diseases are not a problem, powdery mildew can be very serious.

Angular leaf spot or bacterial blight of strawberries is caused by the bacterium Xanthomonas fragariae. In the Midwest, it is the only reported strawberry disease that is caused by a bacterium. The disease was first reported in Minnesota in 1960, and has since been found in other regions of the United States. It appears to be spreading rapidly to many strawberry-growing areas of the world with the importation of planting material. Although the disease has not been a major problem in the Midwest, it can become serious and does represent a potential threat to production. Copper fungicides have been recommended for control of bacterial blight with varying degrees of success when applied in a protectant program. Once the disease is established in the planting there is little or nothing that can be done to control it. Hot, dry weather is the best cure for the disease. Cultivars differ in their susceptibility to the disease. None are completely resistant, but Cavendish, Annapolis, Allstar, Honeoye, and Kent are all highly susceptible.

Symptoms

Typical symptoms of angular leaf spot appear initially as minute, water-soaked lesions on the lower leaf surface (Figure 8). These lesions enlarge to become angular spots, usually delineated by small veins. An important distinguishing characteristic of this disease is that lesions are translucent when viewed with transmitted light, but dark green when viewed with reflected light (Figure 9).

(Figure 8) Angular leaf spot (bacterial blight) symptoms on lower leaf surface. Note the water-soakedEspots.

(Figure 9) Angular leaf spot (bacterial blight) symptoms on upper leaf surface. Note the translucent, yellow spots.

Under moist conditions, lesions often have a viscous bacterial exudate on the lower leaf surface. When it dries, the exudate forms a whitish, scaly film. This exudate or film is an additional characteristic that is useful in the identification of angular leaf spot.

Lesions may coalesce to cover large portions of the leaf. Eventually, lesions become visible on the upper leaf surface as irregular, reddish-brown spots, which are necrotic and opaque to transmitted light. A chlorotic halo may surround the lesion. At this stage, symptoms may be difficult to distinguish from those of common leaf spot and leaf scorch.

Heavily infected leaves may die, especially if major veins are infected. Occasionally, under natural conditions, infection follows the major veins, resulting in veinal water-soaking that may or may not spread to the interveinal regions.

Infection by X. fragariae may become systemic. The pathogen can infect all plant parts except fruits and roots and, in some cases, even the fruits have been infected, apparently only in the tissue adjacent to an infected calyx (fruit cap). Calyx infection can be serious. Infected tissues turn black resulting in unattractive fruit (Figure 10).

(Figure 10) Angular leaf spot (bacterial blight) symptoms on strawberry calyx. Note the brown discoloration and drying.

Disease Development

Inoculum for the primary infection of new growth in the spring comes from infected dead leaves where the pathogen overwintered. X. fragariae may survive for extended periods in dry leaves or in infected leaves buried in the soil. Spread is primarily from infected leaf debris or infected crowns.

Bacteria that exude from lesions under high-moisture conditions may provide secondary inoculum. Bacteria may be disseminated to uninfected plants or leaves by splashing water, such as rain or overhead irrigation. X. fragariae gains entrance into host tissue either passively through wounds or actively as motile cells that swim into natural plant openings by means of drops of dew, gutation fluid, rain, or irrigation water.

Very little is known about the epidemiology of angular leaf spot. Development of the disease is favored by moderate to cool daytime temperatures around 68 F (20 C), low nighttime temperature (near or just below freezing) and high relative humidity. Long periods of precipitation, sprinkler irrigation to protect plants from freezing, or heavy dews in the spring also favor the disease. Young leaf tissue or leaves on healthy, vigorous plants are more likely to become infected than those on diseased or environmentally stressed plants.