What great fungus weather we're having!
Originally published in the Davis Enterprise, May 22, 2003
Thus would Dr. Campbell, my irrepressible plant pathology professor, greet us as he bounded into the lecture hall on humid spring days. Conditions were just right, you see, for spores to float and mycelia to creep from plant to plant, starting barely visible infections whose symptoms would be all too apparent in just a few days. His enthusiasm is, well, infectious, but gardeners may not share it.
Disease infection requires a susceptible host, an infecting organism, and favorable environmental conditions: what is called the Disease Triangle. Then diseases spread very rapidly, causing ugly leaves, defoliation or dieback.
April and the first two weeks of May  were truly "great" fungus weather. Rain storms and cloudy days provided moisture, and the relatively cool temperatures only slowed the spread of disease. A few nights above 50F, and symptoms showed up suddenly. Diseases that normally run their course in late winter got prolonged infection periods, so peach leaf curl and shot hole fungus continued to reinfect new leaves on fruit trees.
The number of fungicides has declined as manufacturers decline to renew registration of these materials due to the limited home garden market. Anyway, folks nowadays don't just grab a chemical to spray for pests and diseases. Constant spraying can cause undesirable side effects, including outbreaks of other pests. We tolerate cosmetic damage, remove infected branches and leaves, rake up the leaves that fall to prevent reinfection, and plant resistant varieties. Preventive spraying with safe materials such as Neem oil can be helpful, and liquid copper sprays can prevent some diseases if applied early.
Some diseases can be prevented with a dormant season spray in the winter, which is relatively safe. Wider spacing of plants, especially roses, can reduce the spread of diseases by encouraging better air movement; fungi love dank environments. Hot dry weather will surely come soon enough. The north wind that came through on Sunday dropped the humidity enough to stop the spread of many diseases.
Many fungi are harmless, just decomposers doing their job. Mushrooms in lawns are the fruiting bodies of various fungi decomposing organic material: compost that was incorporated when the lawn was planted, or the roots of a dead tree--even one which was removed many years ago. Just knock them over and they'll wither away (one customer reports good results practicing his golf swing, sending them over the fence
). They're probably not poisonous, but NEVER taste a mushroom of ANY kind without a good handbook and knowledge of mycology!
Rose bushes get three diseases.
Rust spores are the orange bumps on the bottoms of the leaves, with a small yellow spot on top of the leaf. These spores are heavy, so they only move upward by splashing of sprinklers--or the kind of driving rain we experienced on May 3rd. Pick off affected leaves. Dormant sprays and vigorous pruning in winter reduce overwintering spores. Rust fungi are very host-specific; i.e., the kind that grows on roses isn't the same one that grows on snapdragons.
Downy mildew causes irregular brown spots and yellowing of the leaf. If it spreads into the vein the leaf will drop. Badly infected plants are almost completely defoliated this year. The disease is worse on climbers and miniature roses, mainly because they have denser foliage and we prune them less severely in the winter, so the infection can spread more rapidly on these types. Raking or blowing out the leaves that have fallen makes a big difference. Canes that show dieback should be pruned out, and plants that lose lots of leaves will regrow quickly if given a good feeding. Dry weather will put a stop to this fungus.
Powdery mildew shows up a bit later than downy mildew, and mainly on new growth. It can tolerate drier conditions, so it continues into summer. Some roses get it very badly, while others hardly or never get it at all. Since it attacks new growth, just prune out affected portions as you trim out the blossoms that are done. Consider replacing chronically mildewed varieties with less susceptible types, or look for a sunnier, breezier location for your roses.
Different types of powdery mildew also attack the new growth of many types of plants, from Acer (maples) and Aquilegia (columbine) to zucchini and Zinnias. Japanese maples, Chinese tallow, Euonymus shrubs, Chitalpa trees, Scabiosa, Lady Banks' roses, Mexican evening primrose are examples. Woodier plants will outgrow it. Softer, herbaceous plants can just be cut back, which will rejuvenate them and often stimulate more bloom.
Massive leaf drop and sudden dieback of branches can be disconcerting, caused by various blights. Sycamores and plane trees, and ash trees, get Anthracnose blight every year, but it is causing much worse leaf drop than usual this spring (the sycamore leaves are also getting powdery mildew). It's not practical to spray them and new leaves will eventually grow without blight, so just rake up the leaves. Fertilize young trees to get some new leaves. A different kind of Anthracnose blight causes small branches on maples to die. Pruning these branches out helps to prevent the disease from getting worse.
Serious dieback occurs on certain members of the apple branch of the rose family. Fireblight is a bacterial disease which attacks apples, pears (including Asian pears and some ornamental pears), loquats, Pyracantha, and fruiting quinces. It has a narrow and specific temperature range of 55 - 80F, so it shows up suddenly, spreads virulently, and stops as soon as the days get warm. It can cause several feet of a branch to blacken in a few days, and occurs so suddenly that leaves don't even fall off. It infects through blossoms or young fruit. Immediately prune out blackened growth, cutting well below dead stuff, and throw out infected prunings. Old advice was to dip pruners in bleach between cuts, but research has shown that it is not spread on pruners. Bees with dirty feet are the major culprits. Commercial growers of pears apply fungicides during bloom, often at night to avoid harming bees. Home gardeners can plant resistant varieties.
Flowering pears (Pyrus kawakami) and related shrubs (India hawthorn, Carolina cherry laurel, and other ornamental members of the rose family--notice a pattern here?) get ugly spots and drop leaves. This leaf spot fungus is so severe on Pyrus kawakami that most wholesalers have stopped growing the tree. This disease will recycle back onto new growth as long as weather is moist, so raking up the infected leaves can reduce reinfection. Copper sprays can prevent the spread, but copper leaves a green residue on everything that gets sprayed.
Early blight or bacterial speck cause spots on the leaves of young tomato plants. Both spread steadily up the plant, though these are much more severe in humid climates. Check the young plants when you buy them, and pick off infected leaves.
The sudden death of newly transplanted vegetables or flowers is perplexing. Seedlings collapse rapidly, and stems are brown or black. 20 - 30% of plants may die, while others in the same bed are unaffected. Wholesale growers use fewer fungicides in the production of bedding plants than in years past, and modern fungicides suppress rather than eradicate diseases. If the plants are then held under stressful conditions (cold nights, rainy or cloudy days, sudden heat, improperly watered) before being planted, the diseases will attack individual plants which were already infected, especially during transplant shock. This is why we don't plant the heat lovers (peppers and eggplant; Vinca rosea and Zinnias) before the nights are above 55F. Winter-loving annuals such as pansies get the same problem if planted too early in the fall. Plants that are set too deeply in the soil are especially vulnerable as the fungus attacks through the stem or crown.
The good news is that our dry summer weather curtails most fungus diseases. Good for gardeners, that is--not for the fungus.
© 2008 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
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