Fungal disease is generally most active and aggressive under cooler, damp conditions that favor the vitality of fungal spores. Never-the-less, two particularly troublesome diseases, powdery mildew and late blight, do not follow the classic, wet, early-spring pattern.
Powdery mildew is a fungal disease that leaves a superficial, white, dusty coating on leaves, stems and sometimes the flowers and fruit of a wide variety of plants. The cause is a group of related fungi that each attack a limited number of closely related plants. Powdery mildew tends to create a problem for gardeners in middle to late summer, reducing the strength and damaging the appearance of infected plants, but rarely killing them.
The fungal spores themselves are unusually rich in moisture so, unlike many fungal problems that need moist conditions to take hold, powdery mildew thrives with warmth (temperatures around 70 to 80 F), some humidity and shade. The disease is spread when spores are blown from one plant to the next in cooler, dryer weather and flourishes when some warm humidity is added to the picture.
Powdery mildew can be differentiated from downy mildew, another fungal disease that presents a fuzzy, white growth, by the position of the white coating and the growing environment. Downy mildew grows primarily on the undersides of leaves and in more classic wet, cool, fungal disease conditions.
Not surprisingly, good maintenance practices and smart gardening is the key to healthy plants.
In the herb garden, the gardener needs to use a particularly light hand. The rule of thumb with most herbs is the more you neglect them the better they grow. Most people kill their herbs with kindness by over-watering and over-fertilizing them. If you care too much for your herbs, you will only end up with powdery mildew and rotting leaves.
Powdery mildew is often more unsightly than dangerous to the plant, unless it takes hold to the extent of destroying all its leaves or new growth. If your favorite perennials do get attacked, you have several options. When powdery mildew strikes your peonies late in the season, you can just cut back once the leaves get unsightly. If your bee balm has succumbed and is beyond repair, then cut the foliage back; you will get a new flush of foliage but no more flowers for the rest of the season. If it is a late-season bloomer that you’re set on keeping, then remove the worst stems and apply an environmentally friendly product to the rest of the plant as soon as you notice the powdery mildew. Some options include products that contain neem oil, sulfur or potassium bicarbonate.
To control powdery mildew on rose plants, one of the least-toxic and most effective controls for the fungus is the Cornell formula. The formula is 1 tablespoon of baking soda and 1 tablespoon of light vegetable oil or summer weight horticultural oil added to 1 gallon of water. Shake well and spray both the top and underside of all leaves once a week or following a heavy rainstorm.
Powdery mildew is inhibited by extreme heat and extended periods of rain. It will not overwinter in the garden without a plant host, so good garden hygiene is particularly important if you want the best chance of avoiding a repeat of the fungal infection in succeeding seasons. Unfortunately, some powdery mildews can assume a form that allows them to live on the bark or buds of their victim through the winter (this is the case for roses).
Late blight (Phytophthora infestans) is a devastating fungal problem that thrives with high humidity and cool, rainy weather. It covers a plant with white powdery spores and large green to brown spots that swell to the size of a quarter. The stem is typically marked with brown lesions. This fast-moving disease can decimate a plant within a week. The plants affected are members of the Solinaceae family, primarily potatoes and tomatoes, as well as some relatives, such as nightshade and petunia plants.
In the New York area, one of the disease’s greatest targets, tomatoes, are generally planted between Mother’s Day and Memorial Day when the temperatures are warming up. It is generally only late in the season that they are susceptible to cool, rainy temperatures, which is just what the disease needs to flourish. In bright, warm and sunny weather, the intense rays of ultraviolet light break down the fungal spores and stop the disease in its tracks.
A few good gardening practices during the growing season will help stave off the threat of fungal diseases. Water your garden early in the day so that the foliage is dry by the evening and direct the water to the base of the plant, avoiding the foliage. But we have little control over some outside factors. The spores may enter your unsuspecting garden on a new plant; if host, pathogen and ideal environment are present, disease will follow.
The key to dealing with late blight is fast action so that the fungal spores are not allowed to develop and multiply. Good IPM (Integrated Pest Management) practices help here too: inspect gardens on a regular basis to detect and assess any signs of pest or disease problems, catching them in the early stages before the situation explodes. Since this fungal problem progresses so rapidly, remove affected plants immediately and toss diseased plants into the trash rather than risk the disease surviving the composting process and infecting new areas of the garden.
It is hard to overstate how easily the disturbed fungal spores can be spread from one plant to the entire garden. Cut back infected plants midday, once the foliage has a chance to dry. The bright, ultraviolet light will help to destroy any spores that are stirred up. In any disease situation, never work with wet foliage—you will only intensify the problem as water is an ideal conduit for spores. At the end of the season, remove plant debris from infected locations.
It is generally a good practice to rotate your crops on a three- to four-year rotation, but it is especially imperative after a bout with late blight. Don’t plant any member of the Solanaceae family in your tomato bed the following year—not eggplants, peppers nor potatoes.
Caveat: While it is important to take decisive action in cases such as late blight, it is equally important not to overreact when you see other signs of plant damage. For instance, there are other diseases that affect tomatoes, but they are relatively innocuous. Also, plants experience a general wear and tear as they go through the season and end up with blemishes—our version of bumps and bruises—which are strictly cosmetic.
Westcott's Plant Disease Handbook by 
What's Wrong with My Vegetable Garden? by 
American Horticultural Society Pests and Diseases by 
What's Wrong with My Houseplant? by 
The Organic Gardener's Handbook of Natural Pest and Disease Control by 
