'What's that funny brown spot on my plant?'
by Mary Robson, WSU Area Extension Agent
Looking at leaves can be wonderfuly relaxing, but gardeners often panic when they see bumps, splotches, spots, or other irregularities on their cherished plants. What's happening? And the next question often is, "What do I spray on it to fix the problem?" Answering that question precisely isn't always easy, but some specific guidelines may help.
A plant "disease" is loosely defined as something abnormal going on. Prepare to be surprised--most of the leaf oddities on plants aren't caused by any disease organism. About two-thirds of the time, they are caused by the plant's response to stress: too much water, too little water, sun scorch, temperature troubles, or poor soil conditions.
WSU-trained Master Gardeners look at problems brought in by gardeners, year-round. They've given this service for over two decades, seeing thousands of plants every year, and confirming that only about 1/3 of the plants looking "diseased" have been attacked by a specific disease. Last summer, peonies in my garden were scuffed by a rare hailstorm--the leaves looked as if they were suffering from a disease that left a regular pattern of 1/4-inch scratch marks. The plants did fine and are now blooming normally.
If a plant looks peculiar, first determine that it's growing in the best possible conditions. A rhododendron in too much sun will develop scorched spots on the leaves, looking exactly as if a hot iron touched the leaf. An astilbe growing without sufficient water will curl its fern-like leaves and turn brown. A jungle-born houseplant exposed to cold may show distinct purpling or browning of the leaves. Identify your plant and check what it needs to grow best. Check around the roots to see how water is (or isn't) reaching the plant. Obviously, hauling out a sprayer isn't the first thing to do.
Even if the plant does seem to be one of the 33% or so suffering from a disease attack caused by an organism, spraying still isn't the first necessity. In the maritime Northwest, fungal infestations are the most prevalent disease problems. Relatively few common diseases are bacterial or viral. Most fungus organisms multiply well when leaves remain wet--and the drippy spring of 1996 offered ideal conditions for garden life if one's a fungus. Roses everywhere show blackspot, peonies have botrytis, and cherries show brown rot.
The first line of defense for fungal infections is garden tidiness, disposing of any fallen leaves that have disease problems. Fungus organisms often spend the winter on leaves under a plant (a rose bush, for instance) or in fruit remaining on the plant, as with plum brown rot or mummy berry in blueberry. Carefully prune out and discard affected parts of the plant--don't put them into compost piles. Prune into a bag to avoid dragging diseased parts of the plant through the healthy leaves. If possible, prune when the leaves are dry.
Plants can lose more leaves than gardeners suspect. Up to one-quarter of all the leaves can stop growing or fall off, and if the plant is otherwise in good health, it will generally recover. Another part of dealing with diseased plants is to support the plant with water when needed, and fertilize lightly to provide nutrients for new growth. If the plants in the wrong place, move it. A rose in shade, for instance, won't bloom and will suffer more severely from rose fungal diseases.
So, keeping a clean garden is important, and growing plants well to reduce stress is also important. What if the plant has been affected by an identifiable disease problem, appears likely to lose more than 1/4 of its leaves and is valued and desired in the garden?
First, get a precise identification of the problem. Nurseries and Master Gardener clinics can help. Take a sample of affected and unaffected leaves.
If the problem is a fungal infection, fungicides may help reduce the spread of the problem. To understand how they work, learn a bit about the biology of fungal infections. Many fungi infect by landing a spore on a leaf, having the spore begin to grow, and sending parts of the growing organism down into the leaf, penetrating and breaking the surface. Fungicides, in many cases, act as protectants: They keep the fungal spores from penetrating. In most cases, fungicides won't help much after a full-blown infection is obvious.
But they may help if the infection is light, if affected leaves are removed, and then an appropriate fungicide is applied. Peony botrytis, for instance, can be reduced in its impact by a combination of sanitation and proper fungicide use. Emerging foliage won't be protected, so fungicides (such as sulfur, which is a common one), must be re-applied as new unprotected foliage emerges.
Some fungicides, such as lime sulfur used in the winter on fruit trees, kill some fungal spores before they can germinate in spring.
WSU specialists do not suggest using products with combination fungicide-insecticide ingredients. Roses, for instance, are often treated with such products. The insecticide will kill beneficial insects and will make the product more toxic to children, pets, and other desirable garden visitors.
Some ornamental plants have natural disease‹resistance: The Oregon Sugar Pod Pea is resistant to a common pea virus; rosa rugosa and its hybrids resist powdery mildew attacks; the flowering crabapple called "Evereste" fights off scab attacks. With the interest in reducing pesticide use in the garden, one strategy is to select pest-resisting cultivars of plants when installing new ones.
In summary, keep a clean garden. Identify problems before treating. Install pest-resistant plants whenever possible. Use fungicides carefully, and only when needed.
Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.