of Strawberry Diseases
Strawberry Root Diseases
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.
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
(Figure 11) Plants dying from red stele root rot.
(Figure 12) Root system from a red stele infected
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
(Figure 13) Longitudinal section of a healthy (left) and
red stele infected (right) strawberry root.
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, 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.
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
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 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
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.