Keep your garden healthy
One of the most mysterious things that can happen in your garden is when a plant gets sick. How did that happen? Will it spread? Will all my plants die? How can I get rid of it? The most important thing to understand about disease prevention is the disease triangle (drawing, right). A disease only occurs when three things concur: you have a plant that can get the disease (a host), a pathogen that can attack the plant (such as a fungus, bacterium, or virus), and environmental conditions (such as moisture or drought) to promote the disease. If any one of these things is absent, the disease will not occur, so at least one side of the prevention triangle is knocked off. Instead of waiting for a problem to appear in your garden, consider the best defense against the disease a good offense. Here are 10 ways to keep your plants healthy by removing at least one side of the disease triangle.
1. Inspect plants carefully before purchasing
The easiest way to control the disease in your garden is to avoid introducing it in the first place. Getting sick from a new plant is not a bonus any of us want. One of the hardest things to learn is what a healthy plant looks like, and it's hard to know if something you like is sick.
It's a good idea to collect some books, magazines, and catalogs that show what a healthy model looks like. Do not bring home a plant with dead spots, rotting stems, or insects. These problems can easily spread to your healthy plants and are sometimes difficult to eliminate once established.
In addition to checking the tops of plants, always inspect root quality. One doesn't often see customers doing this at a garden center, but it should be a common sight. Place your hand on the surface of the soil with the stem of the plant between your fingers. Gently invert the pot and shake the plant loosely. To loosen the roots from the pot you need to tap the edge of the pot on a solid surface. The roots should be firm, usually white, and spaced throughout the root ball. Dark or thin roots are not a good sign. Even if the tops look healthy, it's only a matter of time before a rotting root system kills a plant.
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2. Use fully composted yard waste
Not all materials in a compost pile decompose at the same rate. Some items may decompose enough to put in the garden, while others may not. Complete composting creates high temperatures for long periods of time that actually kill any pathogens in the material. Infected plant debris that is not subjected to this process can reintroduce potential diseases to your garden. If you are unsure of the conditions of your compost pile, avoid using yard waste as mulch under sensitive plants and avoid adding infected debris to your pile.
3. Keep an eye on your bugs
Insect damage to plants is more than cosmetic. Viruses and bacteria can often only enter a plant through one type of opening, and bug damage provides that. Some insects actually act as carriers for viruses, spreading them from one plant to the next. Aphids are one of the most common carriers, and thrips transmit Impatiens necrotic spot virus, which has become a serious problem for commercial growers over the past 10 years. Aster yellows (photo, right) is a disease transmitted by aphids and has a wide range of host plants. Insect attacks are another way to stress a plant, making it less likely to resist disease.
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4. Clean up in the fall
Even if you live in a temperate climate, it's always best to clean up the garden in the fall. Not only does this prevent disease, but it's also a great way to control diseases that are already in your garden.
Diseases attack dead leaves and litter over winter and new leaves that appear in spring. Examples of diseases that can be dramatically reduced if dead leaves are removed each fall are iris leaf spots, daytime leaf streaks, and black spots in roses. If you have stems and foliage to create winter interest, be sure to remove them before new growth begins in the spring.
5. Apply the right fertilizer
You need to be careful when fertilizing plants, as too much fertilizer can burn the roots and reduce their ability to absorb water. In turn, plants become more stressed from drought, cold, and heat. Plants that are starved for nutrients are smaller and may be badly affected by leaf spots, while a stronger plant will fight disease. An excess of a particular nutrient is another way to stress a plant.
Getting a soil test through your local extension agency will give you accurate information about the nutrient levels in your soil. Without it, feeding your plants can be guesswork on your part and one nutrient may be too much or another may not be enough.
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6. Varieties of Plant Disease Resistance
Disease-resistant plants can be diseased by a particular problem but fight the disease instead of succumbing to it. For example, some tomatoes are labeled "VFN Resistant," meaning the tomato variety is resistant to Verticillium and Fusarium and nematodes.
If you start looking for these codes in flowers, you'll be disappointed because disease resistance is rarely identified in plant tags. This does not mean that many varieties of flowers are not resistant to disease. Many rose companies offer plants resistant to diseases such as powdery mildew and black spot.
Nursery staff and fellow gardeners can help you identify the best or most resistant varieties of many plants. Reference books and catalogs may also list plants and varieties resistant to specific diseases.
7. Prune damaged limbs timely
It is better to prune trees and shrubs in late winter than to wait until spring. Injured limbs can become infected over winter, allowing the disease to establish while the plant is dormant. Late winter pruning prevents the disease from spreading to new growth. Although late winter storms can cause new damage, it's better to mend broken joints than to ignore them until spring rolls around. Always use sharp instruments to make clean cuts for faster healing and ensure healthily, living tissue is cut back.