Fungal Woes Could Follow Wet Winter Weather

From The Fresno Bee, January 7, 2010, by Elinor Teague:

I'm sure we all have our fingers crossed, hoping that December's welcome rains continue into the spring months.

If the wet weather pattern does persist, though, we may see a return of fungal and bacterial disease problems that have not been as prevalent during the past three drought years.

Fungal spores and bacteria overwinter on leaves and twigs and in cankers on the bark surfaces and can be carried onto susceptible plants by splashing rain.

Anthracnose [picture 1], which is a serious problem for ash and sycamore trees, is one such example of a fungus that is spread in water droplets in the cooler winter months.

Peach leaf curl fungus is another example.

The first twigs and shoots on trees infected by anthracnose fungus turn brown and die; the first spring flush of new leaves show dead brown areas along the veins.

Eventually, the first crop of leaves falls off and is then replaced by another set.

Treatment for anthracnose begins with regular winter cleanup of fallen twigs and leaves as well as pruning to remove cankerous branches.

Consult with a certified arborist before pruning large trees.

Yearly spraying of infected trees with fungicides is not very effective and root injections of systemic fungicides are not recommended by UC Davis. You might want to consider replacing an anthracnose-infected tree with a variety resistant to the disease.

Peach leaf curl [picture 2] infection on peaches and nectarines occurs in early spring as the buds begin to swell but before they break open and show the green lining.

New University of California at Davis guidelines for treatment of peach leaf curl recommend just one spraying of a lime sulfur or a copper-based fungicide in early spring at bud swell. The sprays can be washed off by rain or overhead irrigation, so the fungicide must be applied between rainstorms.

And, of course, clean up fallen leaves and any mummies (dried fruit) that may harbor the fungal spores.

Eutypa dieback on apricots is caused by a fungus that is carried into pruning wounds by rain or irrigation.

The fungus, also called gummosis [picture 3], causes limbs to wilt and then fall of with leaves still attached. The surface bark turns dark brown and an amber, gummy substance may show.

The interior wood is also brown. To prevent eutypa dieback, apricots should be pruned after harvest during the dry season, so in July, August or September in our area. disinfect pruning shears between cuts with a 10% bleach solution and apply a bleach solution to pruning wounds on infected trees.

Olive knot and oleander gall are bacterial diseases that also are carried into pruning wounds on olive trees and oleanders by rain in winter and spring.

Galls, or unusual growths, form around pruning wounds on infected plants and heavily affected branches may die.

These plants should also be pruned in the dry season. Again, prune out dead branches and disinfect tools between cuts.

[For more information and pictures of anthracnose in shade trees, go to University of Illinois' web page.]