The Q&A Archives: Fungus

Question: What causes a fungus on the branches of a Mimosa Tree and causes it to die? Will it spread to others in the same yard?

Answer: Mimosa trees are susceptible to a disease called Fusarium, which causes a disease called wilt. Symptoms of mimosa wilt include chlorosis (leaf yellowing) and leaf wilt by early to midsummer, after which many leaves may yellow and drop without wilting. Some trees die within a few weeks after first wilting, but most die branch by branch over several months. Almost all infected trees die within a year of first wilting. In advanced stages, infected trees ooze a frothy liquid from cracks and grow sprouts on trunks. Brown streaks develop in the roots on the side of the tree where branches first begin to wilt.

The fungus grows into the woody tissue and produces spores that are carried upward in the sap stream. Spores lodge at vessel end walls, germinate, and penetrate adjacent vessels and cells. Before the cells die they secrete a brown, gummy substance to aid in walling off the infection. Too often the secretion is behind the advancing fungus and the tree continues to wilt.

Once the stem is defoliated, the fungus grows from the wood into the bark and produces orange to pinkish fruiting bodies on the bark surface. Fruiting bodies can survive up to two years on a dead trunk and produce masses of canoe-shaped spores (conidia). Spores wash off in irrigation or rain water, and can be transported long distances by surface water.

A balanced fertilizer (10-10-10) may help alleviate symptoms in infected trees; never use high-nitrogen fertilizers. Infected trees should be watered frequently to decrease wilt symptoms, and dead branches should be removed and burned.

Because Fusarium is a vascular wilt pathogen, surface-applied fungicides are not effective. Even with systemic fungicides, chemical control of Fusarium wilt is not practical when treating established trees. The most economical control is to plant resistant cultivars of trees and shrubs.

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