OZONIUM ROOT ROT (ORR): Phymatotrichopsis omnivora (Duggar) Hennebert (also called Phymatotrichum omnivorum Duggar) Fungi: Basidiomycetes: Aphyllophorales

DISTRIBUTION: Southwestern United States, central and northern Mexico (limited distribution in CA). Soil from AZ must be certified ORR free before entering CA.

DESCRIPTION : The causative organism is a soil born fungus. ORR produces tawny yellow fungal (mycelial) strands (200 µm in diameter) with distinctive cruciform (arranged in a cross) branches (Fig 1). The bark is destroyed, and the fungi fill the sap carrying tissue. Following the death of the plant, sclerotia form. Sclerotia are small (1-2 mm in diameter), densely compacted masses of thick-walled cells. The irregular shaped sclerotia are first white, changing to buff, brown and black with age. The fungus can form batter-like spore mats on the soil surface, which are 50-400 mm in diameter and are white to tan in color (Fig 2).

HOSTS: Over 2,300 species of broadleafed plants, including cotton, alfalfa, grapes, fruit trees, sugar beets, and many ornamentals.

DAMAGE: ORR reduces yield by killing cotton plants and partly developed bolls. Lint quality may be reduced in plants that survive until harvest. Stone fruit trees and ornamentals may be killed. Monetary losses from quarantine actions may be significant.

INSPECTION TIPS: The first symptoms may be slight yellowing or bronzing of leaves followed by wilting. Infected plants suddenly wilt and die in the heat of the summer and the dead leaves remain attached to the plant (Fig 3). Large trees and shrubs may die more slowly. A reddish lesion around the crown of the plant may develop on trees killed by the fungus.

 Look for localized dead patches in fields (Fig 4). Affected plants pull from the soil with little effort and root bark sloughs off easily. Using a 10 X hand lens, look for decayed and brownish root bark with strands of fungus on the root surface (Fig 5). A laboratory specialist must make the definitive diagnoses. A root sample with soil attached should be kept cool in a plastic bag (do not add water or paper towels).

LIFE CYCLE : ORR prefers alkaline soil low in organic matter. The fungus generally invades new areas by continually slow growth through the soil from plant to plant. It can also spread through infected transplanted plants. Sclerotia enable the fungus to exist without a host for at least five years and as deep as 12 feet. The spore mates (often confused with other harmless fungi) may appear on soil surface of infested areas, but do not spread the fungus. Spore mats are seldom seen in cotton. Seedlings are not susceptible. Broadleafed plants such as mesquite trees are not susceptible but can act as a source of infection. Grasses and palms are immune.

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