Phytophthora crown rot of strawberry

Phytophthora crown rot is caused mainly by the pathogen Phytophthora cactorum. Strawberry plants infected with Phytophthora crown rot frequently occur in patches of wilted and stunted plants. Leaves of infected plants may appear bluish green initially; however as the disease progresses, leaves on infected plants may develop brown necrotic tissue along the leaf margins and between the veins. The plants wilt very quickly during fruit development or during warm or hot weather conditions when plants are demanding water regardless if the soil contains adequate moisture. Symptoms are often first noticed in low areas of a field or rows where water had accumulated for an extended period after irrigation or a heavy rain. Eventually the disease moves along the rows from the patch of initially infected plants. Cutting open the crowns longitudinally with a knife reveals a red or reddish dark brown spongy rot inside infected crowns (Figure 1). The reddish brown discoloration may appear at the base, middle or top of the crown. Often roots attached to the infected crown appear black at the point of attachment. The root systems are also frequently discolored and may have fewer secondary roots.

The pathogen survives as persistent, resistant, resting oospores for long periods in soil or infected plant tissue. During wet conditions, the resting pores germinate to produce a structure called a sporangium. Under moist conditions, roots and crowns can become infected if they come in contact with the sporangium. However, when soils become very wet or saturated for a sustained period of time (30 minutes to 6 hours), the sporangium produce and release many specialized spores called zoospores that swim toward and infect the crowns and roots of strawberry plants. Strawberry plants growing in poorly drained fields where standing water occurs after a heavy rain; or heavy, wet soils are at more risk of becoming infected by Phytophthora spp. The pathogen can contaminate ponds when run off from contaminated fields flows into the ponds. Irrigation from contaminated ponds can spread the pathogen to other fields. Infected nursery plants can spread Phytophthora spp. into new fields as well as movement of contaminated soil on equipment.

Managing Phytophthora crown rot in strawberries is difficult and requires an integrated approach. Select fields that are well drained with light soils if possible. If well-drained or light soils are not available, plant in raised beds. Avoid planting in fields that had a history of crown rot. Use disease free transplants to avoid introducing the pathogens into your fields.

A new fungicide has recently been registered for Phytophthora crown rot in field strawberries. Orondis Gold (FRAC group 49 + 4, oxathiopiprolin + metalaxyl-M, PCP #33508) is registered for suppression of Phytophthora crown rot and control of red stele. Orondis Gold can be applied twice per season as a banded soil spray or drip application. On new plantings the first application of Orondis Gold should be applied soon after planting. On established plantings the first application should be made before first bloom (PHI 30 days) and a second after harvest in the fall. Apply Orondis Gold in a minimum of 2000 L water/ha to ensure the product reaches the crown and roots.

Phytophthora cactorum can also cause fruit rot (called leather rot) when soil is splashed onto the fruit. Management for leather rot may include fungicides that are effective on crown rot, but placement of fungicides differs. Additionally, Aliette (FRAC group P07, fosetyl-al, PCP#27688) is registered on red stele, caused by Phytophthora fragariae and may reduce Phytophthora crown rot.