Growing Concerns 581: How to battle fungus and mold

Late Summer!

Plants can win the fungus war even if leaves lose some battles


Dear Readers,

You've sent many questions about tree, shrub, flower and vegetable leaves that are spotted, mildewed, yellowing, shriveled and dropping early. Here are excerpts from that mail.

These examples feature various plants and diseases, but there is a common thread. Each involves a leaf-attacking fungus and similar damage assessment principles and control strategies apply to all. If you're facing leaf problems that resemble any of these, use what you read here as a general guide to gauge the seriousness of the condition and devise appropriate treatment.

M.C. writes,

"I have... choke cherry and tart cherry trees. They are dropping leaves like it was fall. I'm worried that they may have some kind of disease. The tart cherry tree leaves are spotted like something is eating them prior to dropping... The choke cherry leaves are just turning yellow and falling off."

Dear M.C.,

They may be infected with one or more of the fungi that prey on the cherry family. Most of these are cosmetic, not life threatening, diseases. View them as acne rather than cancer.

The infection infiltrated the leaves in spring, and grew as temperature, humidity and the plant's internal chemistry allowed. As a leaf reaches the point when it can't benefit the tree any more because too much of its tissue has been consumed by the fungus, the tree sheds it.

Some points of infection mature and produce spores, creating distinct spots where tissue may die and fall out before the leaf drops. Those "shot holes" can look like insect chewing.

Leaf disease is rarely fatal to a tree. Even if every leaf drops in August, they did produce energy for months, all the tree needed for that year and for a good start in spring. See for yourself: Check next year's buds on a bare branch. Breathe easier if buds are plump and of good size, comparable to what you see on a cherry tree that did not defoliate.

M. reports,

"White spots on the leaves of my zucchini and spaghetti squash plants and squash that had been maturing has now rotted."

Dear M.,

The squash family, including pumpkins, cucumbers and zucchini, are susceptible to mildew. To annuals like these, fungi that kill leaves, flowers or fruit are more than just disfiguring. You can remove the infected leaves and use a fungicide to protect new growth, but you can't make up the loss.

So take preventive steps next year. Lay out rows to admit the wind, allow wide spaces between plants or grow the vines on a trellis, all to keep foliage dry and fungus-free. You can also provide airy straw bedding to hold the fruit off the wet ground, pick off yellowing leaves before they go contagiously gray, and try to prevent infection by spraying the most susceptible plants regularly with a fungicide. Switch fungicides to keep the fungus from developing an immunity to any one product.

B.K. sees,

"...Norway maple leaves covered with round yellowish spots that are turning shiny black. This has not affected our other, different variety maples. Will it end when the leaves fall or do we need to have it sprayed?"

Dear B.K.,

That's Rhytisma acerinum, or tar spot. Silver-, red- and sugar maples can be infected but seem more "resistant" -- able to discourage the fungus from maturing into spots.

We call a fungus a cosmetic problem if it affects only expendable parts -- leaves rather than branches, roots or the water-conducting system. We also rate it less seriously when it damages only a portion of each leaf or comes late in the season.

Granted, it's ugly, but your tree won't die and will one day be clean again. Whatever the conditions are that permitted this minor and formerly little-known disease to proliferate suddenly over a huge portion of the U.S., they will change. It may take a few years but the disease will fade again to obscurity.

You can arrange for fungicide sprays but since an entire region and so many trees are involved, including untended forest trees, wind-borne spores will constantly bathe your tree. You probably won't achieve a high degree of control.

R.P. writes,

"I just planted a ....viburnum and am noticing dark spots on all the leaves."

Dear R.P.,

Look for spots on twigs, too. Viburnum leaf spot, which affects just that group of plants, can also infect young twigs. There it grows for years, weakening and eventually killing the limb.

When permanent parts of a plant are affected, or a plant is just getting established, be aggressive in fungus control.

Good sanitation is the most effective tactic against all fungi. Snip off or pick up infected leaves. Hot compost them or send them to a yard waste facility that will do that, because the heat of a compost pile kills most fungi. Spray the remainder of the plant with fungicide.

Encourage the shrub to establish quickly. Make sure it has full sun, loose well drained soil, slow release fertilizer in late fall and steady water next spring. Once it has a large, vigorous root system, the shrub will fend off disease without your help.


Green thumbs up

to birds who gorge on ripe fruit and seem to enjoy their drunkenness, assuring us that people aren't the only foolish creatures.


Green thumbs down

to raccoons, squirrels and groundhogs who can sense a vegetable's ripeness from a distance and be there to harvest it before we can.


Originally published 8/28/04