Excessive rain promotes Phytophthora diseases in raspberries, strawberries

In a rainy year, there is a higher risk of diseases caused by Oomycetes, also called “water molds,” especially Phytophthora species, which thrive in waterlogged soils and areas with standing water for periods of time. They can cause rapid wilting and decline of plants or rotting of fruit. Phytophthora mycelium can survive in infected roots or fruit mummies or as hardy oospores in plant debris and soil. Oospores are known to remain viable in the soil for over 10 years. Oospores germinate under moist conditions, forming balloon-like sporangia that contain motile zoospores. The zoospores swim through the water-filled soil pores to susceptible plant parts and cause infection; wounds are not required. Infection is more likely during cool, rainy periods in fall and spring, but can occur throughout the growing season if conditions are favorable and susceptible host tissue is available.

Phytophthora diseases may be introduced to new sites through infected planting material or spread via runoff from infested fields and movement of soil on boots and equipment. Below are descriptions of different Phytophthora diseases in berry crops and Michigan State University Extension suggestions for management.

Phytophthora root rot of raspberries

Phytophthora root rot of raspberries (Phytophthora megasperma, P. cryptogea, P. citricola and P. cactorum) symptoms include a general lack of vigor and a sparse stand. Apparently, healthy canes suddenly decline and collapse during late spring or early summer. Infected plants frequently occur in patches, which may spread along the row. Because similar symptoms may be caused by other factors, like winter injury, cane borers, etc., suspect plants should be dug up and roots and crowns cut open to look for characteristic, brick-red discoloration and root rot. Rotted roots will eventually turn dark brown as the tissue decays. Sometimes a distinct line can be seen between infected and healthy tissues. Some Phytophthora root rot symptoms have already been seen in raspberries in Michigan.

Red stele of strawberries

Since most strawberries in Michigan are grown on light soils, red stele of strawberries (Phytophthora fragariae) is a relatively rare root disease, but it occurs occasionally in heavier soils. It may also occur in strawberries on plastic-covered beds that are overwatered. Infected plants are stunted and dull green and produce few runners. Older leaves turn prematurely yellow or red, and younger leaves have a metallic bluish-green cast. Plants wilt and die rapidly during the first hot, dry weather of summer. Infected plants have very few new roots and many of the older roots are rotted. When the outside portion of the root is peeled off, the central portion (stele) is brick-red. This is in contrast to black root rot, the most common root problem in Michigan strawberries, where the stele is light in color while the rest of the root is brown to black. The optimum temperature for infection and disease development is 55-60 degrees Fahrenheit, although the pathogen may be active at temperatures as low as 40 F. Under favorable conditions, plants will show disease symptoms within 10 days of infection.

Leather rot of strawberries

Leather rot of strawberries (Phytophthora cactorum), is an important fruit rot disease that may cause considerable losses, sometimes as high as 50 percent. In addition, infected berries have an unpleasant, pungent odor that can be detected in preserves even at low incidence. Excessive rainfall promotes the disease, as well as a lack of straw cover which allows berries soil contact or soil splash. The pathogen attacks fruit at any stage of development, but may also infect blossoms, often killing whole clusters. Infected green berries turn brown and leathery. When ripe berries get infected, they turn pink to light brown and become soft. Under wet conditions, a white, fuzzy growth may be seen on infected berries, containing sporangia that can be rain-splashed to surrounding berries. At the optimum temperature of 62-77 F, only two hours of wetness are needed for infection and the disease can spread very quickly. The berries eventually mummify and overwinter on the ground.

Management

To manage Phytophthora diseases, moisture management is foremost. Chemical fungicides may be used to prevent infection, but will not cure dying plants or rotting fruits. They are best used in an integrated program with other practices, including:

• Selecting a site with good drainage or improving drainage and reducing soil compaction.
• Avoiding planting next to a barn or shed where water from the roof may puddle.
• Planting on raised beds at least 10 inches high; mix a porous material like bark, but not sawdust or peat, into the bed to improve aeration.
• Avoiding previously infested sites or planting resistant cultivars if the site has a history of red stele. Red stele-resistant strawberry cultivars include Allstar, Earliglow, Guardian, Midway, Redchief, Redglow, Scott, Sparkle, Sunrise and Surecrop and day-neutral cultivars Tribute and Tristar. However, none are resistant to all races of the pathogen.
• Rotating out of raspberries or strawberries for five to 10 years; however, the efficacy of this is questionable as the pathogen is very long-lived.
• Cultivating infected fields last, cleaning equipment and reducing runoff from infected areas.
• Using disease-free planting material from a reputable nursery.
• Pre-planting root dips and foliar sprays with a phosphite fungicide (e.g., Aliette, Phostrol) or post-planting ground or drip applications of Ridomil Gold may be advisable. Phosphites can be applied all season while Ridomil Gold is usually applied to the soil in spring and fall.
• Pre-planting soil fumigation with soil sterilants (e.g., Telone C-35) is effective, but expensive and toxic to all soil life, including beneficial microbes.

Specifically for leather rot:

• Apply a layer of straw to prevent berries from touching the soil.
• Plant rows in the direction of the prevailing wind and avoid excessive growth to reduce moisture in the planting.
• Pick fruit frequently and early in the day; remove rotting berries and dispose of them – do not leave them in the field.
• Use effective fungicides, such as phosphites (Aliette, Phostrol, etc.), Ridomil Gold or strobilurins (e.g., Abound and Pristine).

— By Annemiek Schilder, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences