Tips for managing plant health
Plants have defenses similar to a human's immune system, which swing into action when plants are under attack from an insect or disease. If plants are under stress, they cannot react with full strength to fight off or recover from diseases. Stressed plants, therefore, are more likely to succumb to these afflictions.
A disease happens when these three things coincide: a plant that can get sick (this is a host), a pathogen (like a fungus, bacterium or virus) that can attack the plant, and environmental conditions (like humidity or drought) that promotes the disease. This is called a disease triangle. 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.
When buying plants make sure the plant is healthy. It should look full, with good color; if it is a flowering annual it should show lots of buds. The plant should not look stressed, or like it is drying out. Turn the plant upside down and pull the container off the plant to check that the roots are healthy and white.
Cleaning up your garden in the fall is not only an effective deterrent to disease but also a good way to control diseases already in your garden. Diseases can overwinter in dead leaves and debris and attack the new leaves as they emerge in spring. Iris leaf spot, daylily leaf streak and black spot on roses are examples of diseases that can be dramatically reduced if the dead leaves are cleared away each fall. If you leave stems and foliage to crate winter interest, be sure to remove them before new growth begins in the spring.
Insect damage is much more than cosmetic. Viruses and bacteria often enter a plant through some sort of opening, and bug damage provides that. Some insects actually act as transports for viruses spreading them from one plant to the next. Aphids are common carriers. Thrips spread impatiens necrotic spot virus. So if these are present you may need to spray the plant. However it is a better alternative if we can keep these plants healthy, and/or remove a plant as soon as you notice there is a disease problem.
Fertilizer is good, but take care when fertilizing plants that you do not use too much fertilizer as it will burn roots, reducing their ability to absorb water. This in turn makes 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 stronger plants can fight off these diseases.
Watering your garden is a good thing, but diseases need water just as much as plants do, so how we water makes a big difference to our plants. Many pathogens in the soil and air need water to move, grow and reproduce. To avoid giving these diseases an environment they love, choose watering methods that limit moisture on a plant's foliage. Soaker hoses and drip irrigation accomplish this. If you are watering by hand, hold the leaves out of the way as you water the roots. Overhead sprinkling is the least desirable option for watering. However if you choose this method, water early in the day so the foliage will have time to dry off.
Make sure your plant is in the right place. If it is a sun loving plant make sure it is in the sun, and if it is a shade loving plant that it is in the shade. The location will affect how well the plant will thrive and the will result a healthy or stressed plant.
Take care when spacing plants, and keep an eye on established plants as they spread. Crowded plants create their own humidity, which allows diseases like powdery mildew, rust and downy mildew to thrive. Improving airflow around plants reduces high relative humidity and allows foliage to dry more quickly.
Plants close together tend to grow poorly due to competition for light, water and nutrients. These weak plants are more susceptible to attack. Diseases also sometimes spread when an infected leaf comes in contact with a healthy one, which is more likely when plants are close to each other. To lessen the likelihood of disease, trim out crowded, damaged or old stalks on plants, and divide or rearrange your plants when they become crowded.
Kyle Schulz is a Wadena County Master Gardener from Sebeka, and the regular gardening columnist for the Wadena Pioneer Journal.