One of the most mystifying things that can happen in your farm is when a plant gets a disease. How did it 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 something called the disease triangle (drawing, right). Disease can only happen when three things coincide: you have a plant that can get sick (a host), a pathogen (like a fungus, bacterium, or virus) that can attack the plant, and environmental conditions (like humidity or drought) that promote the disease. If any one of these things is not present, the disease will not happen, so prevention involves knocking out at least one side of the triangle. Rather than waiting for a problem to pop up in your farm, consider the best defense against disease to be a good offense. What follows are 6 ways you can eliminate at least one side of the disease triangle and keep your plants healthy.
Examine seedlings carefully before buying
The easiest way to limit disease in your farm is to avoid introducing it in the first place. Getting a disease with a new plant is not the kind of bonus that any of us wants. One of the hardest things to learn is what a healthy plant should look like, making it difficult to know if the one you want is sick.
It is a good idea to collect a few books, magazines, and catalogs that show what a healthy specimen looks like. Don’t take home a seedling with dead spots, rotted stems, or insects. These problems can easily spread to your healthy seedlings and are sometimes hard to get rid of once established.
In addition to checking the tops of seedlings, always inspect the root quality. One does not often see customers doing this when they are purchasing, but it should be a common sight. Place your hand on the soil surface with the plant stem between your fingers. Gently invert the sachet and shake the plant loose. You may have to tap the edge of the sachet against a solid surface to loosen the roots from the pot. Roots should be firm, usually white, and spaced all over the root-ball. Dark or mushy roots are not a good sign. Even when the tops appear healthy, it’s just a matter of time before a rotted root system kills a plant.
Use fully composted yard waste
Not all materials in a compost pile decompose at the same rate. Some materials may have degraded sufficiently to be put in the farm, while others have not. Thorough composting generates high temperatures for extended lengths of time, which actually kill any pathogens in the material. Infected plant debris that has not undergone this process will reintroduce potential diseases into your farm. If you are not sure of the conditions of your compost pile, you should avoid using yard waste as mulch under sensitive plants and avoid including possibly infected debris in your pile.
Keep an eye on pests
Insect damage to plants is much more than cosmetic. Viruses and bacteria often can only enter a plant through some sort of opening, and pest damage provides that. Some insects actually act as a transport for viruses, spreading them from one plant to the next. Aphids are one of the most common carriers, and thrips spread impatiens necrotic spot virus, which has become a serious problem for commercial producers over the past 10 years. Insect attacks are another way to put a plant under stress, rendering it less likely to fend off disease.
Apply the correct fertilizer
You need to take care when fertilizing plants since too much of any fertilizer can burn roots, reducing their ability to absorb water. This, in turn, makes the plants more susceptible to stress from drought, cold, and heat. Plants starved for nutrients are smaller and can be badly affected by leaf spots, while a stronger plant can fight off diseases. An overabundance of a particular nutrient is another way to put stress on a plant.
Getting a soil test through your local extension agency will provide you with accurate information on nutrient levels in your soil. Without it, any feeding of your plants is likely to be guesswork on your part and may result in too much of one nutrient or not enough of another.
Plant disease-resistant varieties
Disease-resistant plants are those that might get sick with a particular problem but will fight off the disease instead of succumbing to it. These are mostly hybrids or F1 seeds/seedlings.
Agrovet employees and fellow farmers in Kenya 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 particular diseases.