Diseases of Grapevine Roots

Armillaria Root Rot

• Armillaria, or oak root rot fungus, thrives in constantly moist soils. The fungus persists for as long as 50 years in the right conditions. It feasts on decaying roots and wood. Soil debris left following the removal of previously infected plants spreads fungal strands to healthy grapevines. Vineyard flooding also distributes active fungi. Symptoms of Armillaria infection include dark threads, or rhizomorphs, extending through the soil between vines. Mushrooms may appear at the bases of the vines in winter. Newly infected plants produce sparse, fragile shoots. Their discolored leaves drop prematurely. Vines with advanced Armillaria cease production. Complete collapse typically occurs in late summer. Death follows infection in as little as one year.

Phytophthora Root Rot

• Like Armillaria, the Phytophthora megasperma root rot fungus flourishes in wet soil. Newly planted grapevines with trunks constantly wet form drip irrigation are especially at risk of Phytophthora. Symptoms include stunted growth and foliage thinning as the infestation advances. Death occurs shortly after the fungus enters a vine's root crown. The disease survives in the soil for years.

Cotton Root Rot

• Grapevines in hot summer climates and alkaline soils with pH readings above 7.4 are susceptible to cotton, or Phymatotrichum, root rot. The Phymatotrichum omnivora fungus spreads in strands through the roots. It produces cottony, bark-decaying growths that invade a grapevine's vascular system. After a vine succumbs, fungal growths called sclerotia appear in the soil strands. They can lie dormant for years between outbreaks. Infected vines become stunted, with wilted, prematurely dropping leaves. Sunburn may damage fruit left unprotected from defoliation. The fungus spreads from vine to vine in a slowly growing, circular pattern. Plants that don't die from winter cold deteriorate throughout the next growing season.

Management

• Grapevine root disease control begins with eliminating the consistently wet soils in which Armillaria and Phytophthora fungi thrive. Planting vines in acidic soils with pH readings below 6.5 and climates where temperatures consistently remain below 82 degrees Fahrenheit prevents cotton root rot. Sites previously planted with cotton or alfalfa have a higher likelihood of harboring cotton root rot fungus. The best choices for such locations are vines grafted to Champanel cultivar rootstock. It's reportedly Phymatotrichum-resistant.