Disease Management - Growing Organic Orchard Fruits - page 514

Most stone fruits are particularly susceptible to Phytophthora and should
be planted on the best-drained soil available in your orchard. Of the pome
fruits, mayhaw and pear are probably the most resistant to the disease, but
they are still best planted on well-drained soil. For apples, avoid M26 root-
stocks on poorly drained sites. For cherries, Mahaleb rootstocks are particu-
larly susceptible to the disease.
Verticillium
Caused by a soilborne fungus ( Verticillium dahliae ), Verticillium can survive
in the soil for many years and has an extremely wide range of host plants.
Infection comes through the roots, but the pathogen moves upward in the
trees through the xylem. Damage comes from the partial blocking of the xy-
lem vessels responsible for transporting water and also from toxins produced
by the pathogen that cause wilting.
Symptoms. Although younger trees are most easily attacked, the disease
can infect older trees. Lower leaves may show discoloration and wilting, and
those symptoms move upward. The foliage thins, and the tree takes on a bare
look. The decline may occur over several years or shoots and twigs can die
quickly, leaving flags of dead leaves attached to the tree. It is not unusual for
a single branch or side of the tree to show symptoms, creating a prominent
flag in the tree. Cutting into the xylem of infected wood with a knife often
shows brown discoloration in the wood.
Management. If possible, do not plant where tomatoes, potatoes, peppers,
eggplant, raspberries, blackberries, or strawberries have grown. They are
common hosts and can facilitate the buildup of Verticillium in the soil. Phlox,
geranium, shepherd's purse, lamb's quarters, and nightshade are other hosts
and should not be allowed in the understory of an orchard. Where Verticilli-
um is a problem, test your soil for fungal propagules before planting. If mi-
crosclerotia are present, you have a risk of the disease.