Strawberry Root Diseases

Red Stele

Red stele is caused by the soil-borne fungus Phytophthora fragariae. Many commercial strawberry cultivars are susceptible to the red stele fungus; however, many varieties have good resistance to several races of the red stele fungus (Table 1). The use of disease resistant varieties and selection of sites with good soil drainage are the key methods of control. This root rot disease has become a serious problem facing strawberry production in the northern two-thirds of the United States. The disease is most destructive in heavy clay soils that are saturated with water during cool weather. Once it becomes established in the field, the red stele fungus can survive in soil up to 13 years. Normally, the disease is prevalent only in the lower or poorly drained areas of the planting; however, it may become fairly well distributed over the entire field, especially during a cool, wet spring. The red stele fungus may become active at a soil temperature of 40 F. However, the optimum soil temperature for growth and disease development is between 55-60 F. Under favorable conditions of high soil moisture and cool temperatures, plants will show typical disease symptoms within 10 days after infection.

Symptoms

When plants start wilting and dying in the more poorly-drained portions of the strawberry field, the cause is very likely red stele disease (Figure 11). Infected plants are stunted, lose their shiny green luster, and produce few runners. Younger leaves often have a metallic bluish-green cast. Older leaves turn prematurely yellow or red. With the first hot, dry weather of early summer, diseased plants wilt rapidly and die. Diseased plants have very few new roots compared to healthy plants that have thick, bushy white roots with many secondary feeder roots (Figure 12). Infected strawberry roots usually appear gray, while the new roots of a healthy plant are yellowish-white.

(Figure 11) Plants dying from red stele root rot.

(Figure 12) Root system from a red stele infected strawberry plant.

The best way to identify the disease is to carefully dig up a wilted plant and peel off the outside portion of several roots. The inside or central portion of the root is known as the stele. If the stele is pink to brick red or brownish red, the plant has the red stele disease (Figure 13). The stele of normal plants is yellowish-white. The red color may show only near the dead tip of the root or it may extend the length of the root. The red stele is best seen in the spring up to the time of fruiting. No other disease of strawberry produces this symptom.

(Figure 13) Longitudinal section of a healthy (left) and red stele infected (right) strawberry root.

Disease Development

The red stele fungus is introduced into new planting sites mainly through the distribution of infected plants. The fungus can be spread within a field or area by anything that carries or moves infested soil (implements, shoes, water, etc.). Once in the field, spores (oospores) of the fungus produce large numbers of smaller spores (zoospores). Zoospores are motile and swim about when soil moisture is high. Zoospores invade the tips of young fleshy roots. Once in the roots, the fungus grows and destroys the water and food conducting tissues resulting in wilting and plant death (Figure 14). As soil temperatures rise, the fungus forms large numbers of oospores in the stele of infected plants. These oospores survive periods of hot, dry, and freezing weather for several years in the soil.

Figure 14: Red stele disease cycle.  Disease cycle of Red Stele Root 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 IPM Disease Identification Sheet No.2

Verticillium Wilt

Verticillium wilt, caused by the soil-borne fungus Verticillium albo-atrum, can be a major factor limiting production. When a plant is severely infected, the probability of it surviving to produce a crop is greatly reduced. The Verticillium fungus can infect nearly 300 different host plants, including many fruits, vegetables, trees, shrubs, and flowers as well as numerous weeds and some field crops. Once it becomes established in the field or garden, it may remain alive for 25 years or longer. Several varieties have resistance to Verticillium wilt. The use of resistant varieties and proper site selection and crop rotation to avoid infested soil are the key methods of control.

Cool, overcast weather interspersed with warm, bright days is most favorable for development of Verticillium wilt. Optimal conditions for infection and disease development occur when soil temperatures are 70 to 75 F.

Many soils in the Midwest contain the Verticillium wilt fungus. The fungus can be introduced into uninfested soil on seeds, tools, farm machinery, and from the soil and roots of transplants.

 
Symptoms

The first symptoms of Verticillium wilt in new strawberry plantings often appear about the time runners begin to form. In older plantings, symptoms usually appear just before picking time. Symptoms on above-ground plant parts may differ with the susceptibility of the cultivar affected. In addition, above-ground symptoms are difficult to differentiate from those caused by other root infecting fungi. Isolation from diseased tissue and culturing the fungus in the laboratory are necessary for positive disease identification.

On infected strawberry plants, the outer and older leaves drop, wilt, turn dry, and become reddish-yellow or dark brown at the margins and between veins (Figure 15). Few new leaves develop and those that do tend to be stunted and may wilt and curl up along the midvein. Severely infected plants may appear stunted and flattened with small yellowish leaves. Brownish-to-bluish black streaks or blotches may appear on the runners or petioles. New roots that grow from the crown are often dwarfed with blackened tips. Brownish streaks may occur within the decaying crown and roots. If the disease is serious, large numbers of plants may wilt and die rapidly (Figure 16). When the disease is not so serious, an occasional plant or several plants scattered over the entire planting may wilt and die.

(Figure 15) Strawberry plant dying from verticillium wilt. Note the outer leaves die first

.

(Figure 16) Strawberry field showing symptoms of verticillium wilt.

Disease Development

The fungus overwinters in soil or plant debris as dormant mycelium or black speck-sized bodies (microsclerotia). These microsclerotia can remain viable in the soil for many years. Under favorable environmental conditions, they germinate and produce thread-like fungal structures (hyphae). Hyphae can penetrate root hairs directly or through breaks or wounds in the rootlets. Once inside the root, the fungus invades and destroys the water-conducting tissue. Destruction of water-conducting tissue results in reduced water uptake by the plant; thus, plants wilt and wither. As fungal colonies get older, they produce microsclerotia in infected host tissue and the disease cycle is completed.

Black Root Rot

Black root rot is the general name for several root disorders which produce similar symptoms. The disorders are not clearly understood and are generally referred to as a root rot complex. Although the exact cause of the black root rot is not known, one or more of the following is thought to be responsible: soil fungi (such as Rhizoctonia and Fusarium); nematodes; winter injury; fertilizer burn; soil compaction; herbicide damage; drought; and excess salt, water or improper soil pH. Black root rot has been found in every strawberry-growing area of the United States. Injured plants may be scattered throughout the planting or localized in one or more areas (Figure 17). A considerable incidence of black root rot has been observed in recent years throughout the Midwest. Once the disease is established (shows up) in the planting, little or nothing can be done to control it.

(Figure 17) Strawberry field showing black root rot symptoms.

To recognize black root rot symptoms, it is necessary to know what a normal root looks like. Newly developed main roots of a normal strawberry plant are pliable and almost white. After several months of growth, they generally become woody and are dark brown to black on the surface. When this dark surface is scraped away, a yellowish-white living core can be seen. Small feeder roots that branch out from the main roots should be white as long as they are active.

Roots affected by black root rot have one or more of the following symptoms: 1) the root system is much smaller than normal; 2) the main roots are spotted with dark patches or zones (Figure 18); 3) the feeder roots are lacking or are spotted with dark patches or zones; 4) all or part of the main root is dead (Figure 19). A cross-section of the dead root shows it blackened throughout. Plants with black root rot are less vigorous than normal plants and produce fewer runner plants. Severely affected plants usually die.

(Figure 18) Strawberry root with black root rot symptoms. Note black discoloration (lesions) on the root.

(Figure 19) Strawberry root system with advanced stages of black root rot. Note the dead, black "Rat-tail" roots.