Integrated Management of Strawberry Diseases

Strawberry Fruit Rots

Botrytis Fruit Rot (Gray Mold)

One of the most serious and common fruit rot diseases of strawberry is gray mold. Gray mold is caused by the fungus Botrytis cinerea. Under favorable environmental conditions for disease development, serious losses can occur. The gray mold fungus can infect petals, flower stalks (pedicels), fruit caps, and fruit. During wet springs no other disease causes a greater threat to flowers and fruit. The disease is most severe during prolonged rainy and cloudy periods during bloom and harvest. Abundant gray-brown, fluffy, fungal growth on infected tissue is responsible for the disease's name "gray mold".

During wet, cool springs, gray mold will be a major threat to organic strawberry production. In conventional production systems, application of fungicide during bloom generally results in good disease control. Fungicides used in organic systems (sulfur and copper) are not very effective for control of Botrytis. Several biological control products are currently available for Botrytis control; however, their effectiveness under moderate to heavy disease pressure is questionable. Resistance is not available in most varieties; therefore, the use of several cultural practices are the key control methods in organic plantings.

Symptoms

Young blossoms are very susceptible to infection. One to several blossoms in a cluster may show blasting (browning and drying) that may spread down the pedicel. Fruit infections usually appear as soft, light brown, rapidly enlarging areas on the fruit (Figure 20). If it remains on the plant, the berry usually dries up, "mummifies", and becomes covered with a gray, dusty powder (Figure 21). Fruit infection is most severe in well-protected, shaded areas of the plant where the humidity is higher and air movement is reduced. Berries resting on soil or touching another decayed berry or a dead leaf in dense foliage are most commonly affected. The disease may develop on young (green) fruits, but symptoms are more common as they mature. Often, the disease is not detected until berry picking time. During harvest, the handling of infected fruit will spread the fungus to healthy ones. After picking, mature fruits are extremely susceptible to gray mold, especially if bruised or wounded. Under favorable conditions for disease development, healthy berries may become a rotted mass within 48 hours after picking.

(Figure 20) Immature strawberry fruit with symptoms of Botrytis fruit rot (gray mold). Note the symptoms usually develop first on the calyx end of the fruit.

(Figure 21) Botrytis fruit rot (gray mold) on a mature strawberry fruit.

Disease Development

The fungus is capable of infecting a great number of different plants. It overwinters as minute, black, fungal bodies (sclerotia) and/or mycelium in plant debris, such as dead strawberry leaves in the row. In early spring, these fungal bodies produce large numbers of microscopic spores (conidia), which are spread by wind throughout the planting. They are deposited on blossoms and other plant parts where they germinate in a film of moisture. Infection occurs within a few hours (Figure 22).

(Figure 22) Disease cycle of gray mold on strawberry. We wish to thank the New York State Agriculture Experiment Station for the use of this figure. Figure taken from the Small Fruit IPM Disease Identification Sheet No. 1

Disease development is favored by wet conditions accompanied by temperatures between 41 F and 86 F. Conditions that keep flowers and fruit wet, such as rain, dew, or sprinkler irrigation encourage Botrytis rot.

Strawberries are susceptible to Botrytis during bloom and again as fruits ripen. During the blossom blight phase of the disease, the fungus colonizes senescing flower parts, turning the blossoms brown. The fungus usually enters the fruit through flower parts, where it remains inactive (latent) within the tissues of infected green fruits. As the fruit matures, the fungus becomes active and rots the fruit. Thus, while infection actually occurs during bloom, symptoms are usually not observed until harvest. This is important to remember when one considers control. Temperatures between 70 and 80 F and moisture on the foliage from rain, dew, fog, or irrigation are ideal conditions for disease development.

Leather Rot

Leather rot is caused by the soil-borne fungus Phytophthora cactorum. Leather rot has been reported in many regions throughout the United States. In many areas, it is considered a minor disease of little economic importance. However, excessive rainfall during May, June, and July can lead to severe fruit losses and quality reduction. In 1981, many commercial growers in the Midwest lost up to 50 percent of their crop to leather rot. The leather rot fungus primarily attacks the fruit but may also infect blossoms. Organic fungicides (sulfur and copper) are not effective for control. The key control methods in organic as well as conventional systems are maintaining a good layer of straw mulch between fruit and the soil, and site selection or improvement for good water drainage (avoid saturated soil).

Symptoms

The leather rot pathogen can infect berries at any stage of development. When the disease is serious, infection of green fruit is common. On green berries, diseased areas may be dark brown or natural green outlined by a brown margin (Figure 23). As the rot spreads, the entire berry becomes brown, maintains a rough texture, and is leathery in appearance. The disease is more difficult to detect on ripe fruit. On fully mature berries, symptoms may range from little color change to discoloration that is brown to dark purple (Figure 24). Generally, infected mature fruit is dull in color and is not shiny or glossy. Infected ripe fruit are usually softer to the touch than healthy fruit. When diseased berries are cut across, a marked darkening of the water-conducting system to each seed can be observed. In later stages of decay, mature fruits also become tough and leathery. Occasionally, a white moldy growth can be observed on the surface of infected fruit. In time, infected fruit dry up to form stiff, shriveled mummies.

(Figure 23) Leather rot symptoms on an immature strawberry fruit.

(Figure 24) Leather rot symptoms on a mature strawberry fruit. Note the purplish discoloration.

Berries that are affected by leather rot have a distinctive and very unpleasant odor and taste. Even healthy tissue on a slightly rotted berry is bitter. This presents a special problem to growers in pick-your-own operations. An infected mature berry with little color change may appear normal and be picked and processed with healthy berries. Consumers have complained of bitter tasting jam or jelly made with berries from fields where leather rot was a problem. Leather rot is most commonly observed in poorly-drained areas where there is or has been free-standing water or on berries in direct contact with the soil.

Disease Development

The fungus survives the winter as thick-walled resting spores, called oospores that form within infected fruit as they mummify (Figure 25). These oospores can remain viable in soil for long periods of time. In the spring, oospores germinate in the presence of free water and produce a second type of spore called a sporangium. A third type of spore called a zoospore is produced inside the sporangium. Up to 50 zoospores may be produced inside one sporangium. The zoospores have "tails" and can swim in a film of water. In the presence of free water on the fruit surface, the zoospores germinate and infect the fruit. In later stages of disease development, sporangia are produced on the surface of infected fruit under moist conditions.

(Figure 25) Disease cycle of leather rot on strawberry. We wish to thank the New York State Agriculture Experiment Station for use of this figure. Figure taken from the Small Fruit Crop IPM Disease Identification Sheet No. 4

 

The disease is spread by splashing or wind-blown water from rain or overhead irrigation. Sporangia and/or zoospores are carried in water from the surface of the infected fruit to healthy fruit where new infections occur. Under the proper environmental conditions, the disease can spread very quickly. A wet period (free water on fruit surface) of two hours is sufficient for infection. The optimum temperatures for infection are between 62 and 77 F. As the length of the wet period increases, the temperature range at which infection can occur becomes much broader. As infected fruit dry up and mummify, they fall to the ground and lie at or slightly below the soil surface. Oospores formed within the mummified fruit enables the fungus to survive the winter and cause new infections the following year, thus, completing the disease cycle.

Strawberry Anthracnose

 
Anthracnose is a disease that can affect foliage, runners, crowns and fruit. Various forms of anthracnose can be caused by several fungi. In the Midwest, the most common form of the disease is fruit rot, caused by the fungus Colletotrichum acutatum. Although the disease is not very common, if it becomes established in the planting, serious losses can occur. Organic fungicides (sulfur and copper) are not effective for control. Midwest varieties with resistance are not available. Once the disease develops on fruit in the planting there is little that can be done to control it. Managing the movement of pickers into and out of infested areas and adjusting irrigation practices can be beneficial in preventing disease spread.

Symptoms

Affected stems are sometimes girdled by lesions, causing individual leaves or entire daughter plants to wilt. Under warm, humid conditions, salmon-colored masses of spores may form on anthracnose lesions.

When crown tissue is infected and becomes decayed, the entire plant may wilt and die. When infected crowns are sliced open, internal tissue is firm and reddish brown. Crown tissue may be uniformly discolored or streaked with brown.

On fruit, symptoms first appear as whitish, water soaked lesions up to 1/8 inch in diameter which turn brown and enlarge within 2 to 3 days to involve most of the fruit (Figure 26). Lesions are covered with salmon-colored spore masses. Infected fruit eventually dry down to form hard, black, shriveled mummies. Fruit can be infected at any stage of development.

(Figure 26) Anthracnose lesion on strawberry fruit.

Disease Development

The disease is probably introduced into new plantings on infected plants. Spore production, spore germination, and infection of strawberry fruits are favored by warm, humid weather and by rain. Spores require free water on the plant surface in order to germinate and infect. Anthracnose fruit rot is considered to be a warm-weather disease with an optimum temperature for disease development near 80 F. Thus, the disease is generally a problem in the Midwest when abnormally high temperatures and rainfall occur during fruit set and harvest. Spores are dispersed primarily by water splash. Once the disease is established in the field, the fungus can overwinter on infected plant debris, primarily old-infected, mummified fruit.