Phytophthora Root Rot of Blueberry

W.O. Cline, Extension Plant Pathologist

 

Most highbush cultivars are suceptible to phytophthora root rot; however, the disease is normally of minor importance unless plants are grown in wet soils brought about by poor drainage. Rabbiteye cultivars are more tolerant than highbush to root rot. 

Symptoms and Disease Cycle



Figure 1. New leaves are small and defoliation occurs from root rot.



Figure 2. Size of infected area can be marked by plant death and the presence of water-loving plants.



Figure 3. A field with heavy infection. The improved drainage is helping plants survive.
Phytophthora is soil-borne and root infection is favored by saturated soil moisture. Zoospores (swimming spores) produced by the fungus infect blueberry roots. As the infection spreads, roots collapse and decay. Early above-ground symptoms include yellowing of leaves with some burning of the margins and lack of new growth. As the disease progresses, terminal leaves (Figure 1) become small, and excessive defoliation occurs because of severely damaged roots.

On chronically infected older bushes, nearly all the leaves on the bush can be stunted and wilt-prone. Affected plants will often have a restricted root system that allows them to be easily rocked back and forth or pulled up. Plants in low areas in affected fields are characterized by dead or prematurely defoliating bushes.

Defoliation and poor growth follow the contours of the low areas where excessive soil moisture is present. The presence of sedges (Figure 2) and other water-loving plants is an indicator of the size and shape of these areas.

Control
Lasting control of phytophthora root rot on blueberries is accomplished by improving drainage (Figure 3). Adequate ditching and raised, single-bedded rows are usually sufficient. Rows are most easily bedded up prior to planting. In existing fields, taper-disking or sweep-blading can be used. Field drains using "sock pipe" and other types of pipe normally used for residental septic fields have been effective for draining small areas. Homeowners and pick-your-own growers often encounter problems on wet clay or clay loam soils; the problem is often diagnosed as "planted too deep". On these sites, growers should incorporate peat or bark mulch before planting, then plant shallow and use additional mulch to form beds.

 Other Root Rots

Other fungi, chiefly Armillaria species, occasionally cause root rot on blueberries. If Armillaria is present, a white layer of fungal growth can usually be found just under the bark near the soil. This fungal layer is not present with Phytophthora root rot, and its presence or absence can be used to help distinguish between the two types of root rot. Armillaria rot is most likely to occur in newly cleared areas that have lots of old tree roots remaining in the soil. Control is aided by removal of old dead roots and stumps or by simply not planting in newly cleared areas.

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Last updated: 5 June 1997
Webpage reformatted Dec. 2000