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Problems that crop up as harvest nears

Vegetable and flower gardens can be a joy or a disappointment. In August some common problems reach an ugly climax. Suddenly your tomatoes or zinnias show awful symptoms and go down quickly.

The truth is, none of these really happened suddenly. You just didn't catch the early clues from Mother Nature.

Let's identify some common maladies, consider prevention and what you might do now. All remedies are science-based solutions using IPM -- integrated pest management -- premises, as taught by land grant colleges such as Cornell and Penn State universities.

Black, leathery tomato bottoms: This is called Blossom End Rot, and it happens on tomatoes, peppers, eggplant -- and sometimes vine crops like zucchini. You'll see a dark, leathery area on the bottom, opposite the stem, where the blossom once was.

It is not really a disease. It's a deformity, or growth defect, that results from a lack of calcium when the plant was flowering. But don't go out to buy calcium. The problem was that the plants received too much or too little water during the blossom stage, so they couldn't absorb the calcium needed. Excessively wet, cold or dry conditions shut down moisture and nutrient uptake. The tomatoes planted early are most likely to experience these conditions.

What to do: The condition doesn't hurt you or threaten any other plants or next year's crop. You can cut off the bad parts and use the tomatoes. There is no cure or spray. The plants will probably produce without the condition.

Next year, provide water evenly during the critical blossoming periods, especially with containers or raised beds. Consider planting later, when soil is warm.

Tomato leaves die from the bottom up: The gardener usually describes the plants suddenly dying from the ground up, the leaves showing spots (concentric rings, like a target). Later potatoes or tomatoes (sometimes eggplants, peppers and celery) have lesions, cracks and decaying areas.

It's a fungus called Alternaria Leaf Spot or Early Blight. When conditions are right -- periods of warm temperatures and moist, humid conditions -- the spores multiply, and you'll have it.

What to do: Not much, now for these plants, except to remove and destroy the affected leaves. You'll still get most of the crop. Avoid spreading it by moving around the wet garden. Water the soil, not the plant leaves. Water early in the day so the plants dry out. Clean up well for next year.

Next year, especially in crowded gardens without much wind, raise or stake tomatoes. For critical situations, a copper-based fungicide may help, but for home gardens it's often too late or not worth it.

Looks like the plants were torched: The description points directly to Late Blight (Phytophtora infestans fungus), a serious disease of tomato and potato crops. It caused the 1840s Irish potato famine and has devastated entire commercial crops in some years. The spores can travel hundreds of miles, so home gardeners must identify the disease when it occurs and do the right thing: prevent the spread.

The disease requires these specific conditions: rainy, foggy periods with nighttime temperatures of 40 to 60 degrees, and daytime temps 70 to 80 degrees. It overwinters only in plant tissue, not your soil. Usually it hangs around in plant debris, piles of rotting potatoes or returns next year in free "volunteer" plants that pop up from last year's seeds in the ground.

Besides the "torched" look, you might see dark, leathery spots on tomatoes or dark, water-soaked patches on the leaves (and later white, downy growth).

What to do: Don't take this one lightly. Destroy the entire plants, as well as all plant debris in the garden, putting them into the garbage or burying them deeply. Do not compost. Next year, destroy volunteer tomatoes or potatoes. Don't spread spores around the garden by your watering habits. Space plants well. For important or commercial crops, use an aggressive fungicide program with a product labeled for phytophtera. (See www.hort.cornell.edu/lateblight.)

White, powdery coating on leaves: Powdery Mildew is common on lilacs, ninebarks, roses, phlox, zinnias, bee balm, beans, squash and many other plants. It looks like white or grayish-white powder or white mold. This fungus disease has different strains for specific plants, so your lilac mildew doesn't jump over to the tomatoes, but how you treat the problem is the same.

The spores travel by wind and are everywhere, so if weather conditions are right (high humidity, especially when soil is dry) you'll get it. Vegetable crops develop powdery mildew when the temperatures are warmer than 80 degrees.

What to do: You can't prevent powdery mildew, although some cultivars (phlox, roses) are somewhat resistant. To minimize the disease spread, remove any coated leaves or whole plants, and don't brush through a wet garden. Wash hands, gloves and pruners. Increase the air flow by thinning the garden and weeding. Water early in the day and don't wet the plants.

You can wash lots of powdery mildew off plants, as long as they can they dry out. Fungicides, horticultural oil -- sulfur or copper-based -- may help if you get an early start.

These examples illustrate some key principles of disease management:

* The Disease Triangle: Every disease requires (1) the disease organism (such as spores), (2) specific weather conditions and (3) specific, vulnerable plants. Remove one element = No disease. (Dry, cool spring = No powdery mildew.)

* Accurate identification is everything.

* Most diseases are specific to plant species. Tomato early blight won't hurt perennials.

* Diseases are often out of our control. Some just blow in. We can, and often have to, live with it. That's gardening.

Sally Cunningham is a garden writer, lecturer and consultant.

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