Focus on Fungus: Managing Garden Diseases

[American Beauty Gardener]

Gardeners in our region no doubt welcomed the recent rains. I know I
did. But although they provided only a drop in the bucket when it
comes to replenishing wells, lakes, and aquifers, the rains did bring
something else: garden diseases. Or at least they created the
conditions many diseases need to flourish.

How Diseases Spread
Most garden diseases are caused by fungi, microscopic relatives of the
common garden mushroom. Mature fungi release millions of spores that
are then carried on the wind or otherwise get transferred to our
plants. And if the conditions suit them, the spores germinate and
penetrate leaf tissue, creating spots, lesions, or other symptoms.
Most fungi need moisture to germinate, so damp leaves are an open
invitation. Some diseases are soil-borne and are transported to
foliage when raindrops splash soil onto plants.

Fungi aren't the only disease-causing microorganisms. Some diseases,
such as fire blight, are caused by bacteria. Others, such as tobacco
mosaic, are caused by viruses. Bacterial and viral diseases are
difficult, if not impossible, to control in the home garden.
Fortunately, there are cultural techniques and effective products to
help us control the more common fungal diseases.

Types of Diseases and Symptoms

Leaf spots. The most familiar disease symptoms are leaf spots. You can
often identify the particular pathogen by the shape, color, and
margins of the spots. But it really isn't necessary. The same general
prevention and control measures apply whether you're dealing with
black spot, anthracnose, downy mildew, early blight, late blight,
septoria, or any of the other leaf spot diseases.

Powdery mildew. Aptly named, powdery mildew gives leaves the
appearance that they've been dusted with talcum powder. Unlike the
diseases above, powdery mildew spores do not need moisture to
germinate. The disease, which attacks tall phlox, bee balm, lilacs,
and cucurbits like squash and cucumbers, is worst in hot, dry
weather.

Vascular diseases. Some diseases, such as fusarium and verticillium
wilts, penetrate plant tissues and take up residence in the veins.
Once inside, they clog the veins, preventing water transport which, in
turn, causes plants to wilt.

Diseases on fruits. Many of the diseases that cause leaf spots will
also affect fruits of all types -- apples, strawberries, tomatoes,
cucumbers. Common symptoms include spots, corky areas, sunken patches,
and discoloration.

Flower diseases. If flowers wilt or fail to open, gray mold, caused by
the botrytis fungus, may be the culprit. It's especially prevalent
during periods of high humidity and cool temperatures. Affecting
foliage and flowers, gray mold begins as water-soaked spots which
eventually grow a fuzzy gray coating.

Root problems. "Root rot" is a collective term for diseases that
attack roots. In some cases, the problem is caused by overwatering,
which inhibits the availability of oxygen to roots, causing them to
die and decay.

Abiotic diseases. Herbicide damage, fertilizer burn, drought,
chilling, and nutrient deficiency can cause symptoms similar to fungal
diseases. Rule out abiotic diseases (those caused by something other
than a living organism) before spraying for pests.

Disease Prevention
Simple cultural controls can go a long way toward preventing fungal
diseases.

1. Choose resistant varieties. Plant breeders have created varieties
with built-in resistance to certain diseases. For example, look for
tomato varieties with V, F, or T after the names; these indicate
resistance to verticillium, fusarium, and tobacco mosaic diseases.

2. Space plants properly. Adequate spacing allows good air
circulation, which helps keep foliage dry.

3. Water the soil, not the foliage. Drip irrigation and soaker hoses
are better than overhead sprinklers.

4. Apply mulch. Soil-borne diseases can be thwarted with a layer of
mulch, which prevents soil from splashing onto leaves. Apply fresh
mulch annually under disease-prone plants.

5. Rotate crops. Plants in the same family are often attacked by the
same diseases, so rotate annual crops. For example, avoid planting
solanaceous plants, including tomato, pepper, potato, and eggplant, in
the same garden bed year after year. Rather, rotate with plants in a
different family, such as cucurbits (squash, cucumber, pumpkin).

6. Remove and destroy diseased plant tissue. If only a few leavs are
affected, pick them off and discard (don't compost; composting may not
kill the disease organims).

7. Don't overfertilize.

Deciding When and What to Spray

Consider the plant, the time of year, and the degree of damage before
resorting to pesticide sprays. A small amount of damage on tomato
foliage at the end of the season may be tolerable, while the same
damage in early summer may warrant control. Diseases can spread
quickly if not contained, so observe plants frequently. Note that most
plant diseases are host-specific; that is, they attack similar plants
or plants in the same family. For example, the powdery mildew species
that attacks tall phlox is different than the one that attacks
cucumbers.

Synthetic fungicides have proven to be some of the most toxic lawn and
garden chemicals available. Indeed, several have been taken off the
market, including once-common benomyl, captan, and daconil.
Fortunately, there are now organic options and these, combined with
cultural controls, are very effective. Many organic sprays act by
making plant surfaces unfavorable for disease spore germination.
Here's a rundown.

Copper-based fungicides. A staple for organic gardeners for
generations, copper fungicides are effective but must be used
judiciously because excess copper can harm soil life.

Sulfur-based fungicides. Another important fungicide for organic
growers, sulfur is effective but it comes with some cautions. It is
incompatible with some insecticides, especially oils, and can harm
plants if applied in hot, dry weather. Repeated use can lower soil pH.
Also, copper and Neem Oil have been known to have interaction issues
on plant surfaces.

Neem oil. Derived from the neem tree, this botanical extract is a
multipurpose insecticide, miticide, and fungicide. Use with care
because the oil can harm some beneficial insects. Pure Neem Oil,
Einstein Oil & Bio-Neem are easy-to-use products.

Biocontrols. Serenade is the trade name for a new biocontrol for
several fungal diseases and is based on the bacteria Bacillus
subtilis. It works by boosting the plants' natural immune system as
well as by inhibiting fungal germination and growth. Hydroguard,
another product, contains a naturally occurring patented strain of
Bacillus Subtilis and is especially effective on pythium, as well as
powdery mildews. Tarantula, is a combination of many beneficial
bacillus, as well as plant pathogen immune response boosters.

Bicarbonates. Gardeners have been using homemade baking soda (sodium
bicarbonate) sprays for decades, but recent research has shown that
potassium bicarbonate is more effective at controlling plant diseases.

Compost teas. In addition to providing nutrients, some research is
showing that compost tea sprays can inhibit plant diseases.

Happy Gardening!
Alan, Pete & Mindy @ American Beauty Garden Center

(this information was derived from http://www.garden.org/regional/report/arch/inmygarden/2823
copyright 2008)