Plant Disease Management Handbook

Cause Pratylenchus vulnus is a nematode that lives in soil but is rarely found in Oregon. Root-lesion nematodes are migratory endoparasites where part of the population is in soil and part in the roots at all times. All walnut rootstocks are susceptible but trees on Paradox hybrid are sometimes more vigorous than nearby trees on black walnut rootstock. The rootstock VX211 is tolerant. Can cause serious replant problems so be sure to sample before replanting into an area that has had walnut.

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Hazelnut (Corylus avellana) - Mosses and Lichens

Cause The fungus Microstroma juglandis is widespread in western production areas. The disease is more common in wet years and in orchards with poor air circulation. Hyphae of the fungus emerge from stomata, especially along veins and produce white masses of spores.

Cause The bacteria Rhizobium radiobacter (formerly Agrobacterium tumefaciens) and R. rhizogenes (formerly A. rhizogenes). As galls in soil break down, bacteria are released and can be transported by moving soil or water. Pathogenic bacteria can also be spread by pruning equipment.

Cause Xanthomonas arboricola pv. juglandis (formerly Xanthomonas campestris pv. juglandis), a bacterium that overwinters in infected buds and catkins. Buds with the highest bacterial populations are the ones most likely to develop blight. During early spring growth, bacteria spread along developing shoots and nuts. There seems to be very little secondary spread to other shoots and trees by raindrip.

Cause A naturally occurring chemical, called juglone, found in black walnuts is toxic to certain plants. Highest concentrations are found in buds, nut hulls and roots. The chemical is not very soluble and accumulates under the canopy of trees. However, damage to pine was noted well beyond the dripline of walnut trees. It is also released by living roots in small amounts and by dead roots as they decay. Other nut trees (such as English walnut, pecan and butternut) produce smaller amounts of juglone.

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Cherry (Prunus spp.) - Virus-induced Cherry Decline

Cause A fungus, Ophiognomonia leptostyla. The disease has caused severe defoliation, especially in the 'Adams' variety. The fungus overwinters in old infected leaves and nuts on the ground and in cankers on twigs of the past year's growth. During spring rains, spores (ascospores) are forcibly ejected from fruiting bodies (perithecia), which develop in the old diseased leaves and nuts, and are blown to new growth where they induce primary infection.

Cause The fungus Puccinia vincae has been found in all PNW states. An alternate host is not needed for this autoecious rust. The frequent occurrence of the rust on new leaves and other factors suggest that the rust may be systemic in the plant.

Cause Rhizoctonia solani, a soilborne fungus. It survives in the soil as sclerotia or as mycelium colonizing bits of organic matter. Roots become infected as they grow past these structures. The fungus then grows along root surface, forms an aggregation called an infection cushion, pushes into the root where it colonies, spreads and kills the tissue. The fungus can form knots of mycelium that develop into sclerotia. These are released into the soil as the plant debris decomposes.