For many gardeners, diseases that infect their plants are the bane of their gardening experience. This can include viruses (for which there are no cures; focus is on preventing their spread), fungi, and bacteria. They can fly in with an insect, be harbored on a nearby host plant (even weeds), or spread on the breeze. Community gardens, through plant density and proximity alone, “share” and can even encourage the spreading of diseases. Prevention is easier than curing, and we need to be extra vigilant that we are employing the smartest and healthiest gardening practices that we can to avoid them. Plus, the herbicides that may be recommended for treating a given disease may not be able to be used at Dowling.
Disease prevention literally begins in the previous gardening season when cleanliness and sanitation principles are employed – more on that later, however. When preparing for spring planting we should be mindful of selecting disease resistant varieties. Often seed packets will disclose information concerning disease resistance. With plants you may need to do a little internet research but it’s often readily available when searching by cultivar name (for example, ‘Cherokee Purple,’ an heirloom tomato cultivar, is resistant to Bacterial Speck, Fusarium Wilt 1, Fusarium Wilt 2, Fusarium Wilt 3, Root Knot Nematode, Tomato Spotted Wilt Virus and Verticillium Wilt, according to Cornell University).
Resistance doesn’t mean immunity, and there may be varying levels of protection (partial, moderate, etc.), but it’s the next best thing. It's often hard to identify specific diseases, so having resistance to multiple diseases is generally a good thing. However, if you have positively identified a specific disease in your garden look for resistance to that particular disease.
Use only clean seed. If you save your own seeds, treat them with hot water before using (https://hort.extension.wisc.edu/articles/hot-water-seed-treatment-for-disease-management/). Bacterial speck, bacterial spot and black rot of crucifers, among others, can be brought into a garden via contaminated seed.
Maintaining healthy, strong plants will help them resist diseases. Plants that are already stressed by drought, overwatering, crowding, weed infestation, bad soil, lack of sun or pest damage cannot withstand the onslaught of disease as readily. Weeds can also be hosts for diseases. Good air circulation can sometimes improve plant health by using trellises.
Row covers are an excellent way to greatly reduce if not eliminate the risk of diseases and pest damage when used correctly. However, row covers cannot overcome soil-borne diseases or viruses. By pulling up and destroying plants that are diseased as soon as you see them, the chances of microorganisms persisting in the soil potentially over several seasons will be much less.
General good management practices for disease prevention are way more important than being able to name the top ten diseases you are trying to avoid. They include don’t water in the evening, try to not get leaves wet, mulch to keep pathogens from splashing upward when watering, keep the weeds at bay, don’t work in the garden when it’s wet, and maintain good air circulation around your plants. Rotate your crops!
Sanitation is an important factor – remove and destroy diseased plants (DON’T COMPOST), clean up your garden at the end of the season and remove dead plant material immediately to prevent diseases from overwintering, keep your tools clean (shovels/forks wiped clean of dirt when put away, pruners sanitized after trimming diseased plant material).
If you are offered sickly, unhealthy looking free plants, don’t even touch them to avoid spreading disease.
Wash your gloves and garden clothing frequently and if possible, use shoes dedicated for the garden only.
Wash your hands after using tobacco and before touching plants (tobacco mosaic virus can be transmitted from tobacco products).
There are many diseases of greater or lesser consequence, and some have shown up at Dowling. Just a few include black rot, alternaria leaf spot and head rot (brassicas); anthracnose (melons); phytophthora (peppers and pumpkins), and purple blotch (onions). Tomatoes fall prey to a lot of diseases, such as septoria, early blight, fusarium, bacterial leaf spot, bacterial leaf speck and tomato mosaic virus among many others. Raspberries carry common diseases like botrytis fruit rot, cane blights, phytophthora crown and root rot, and powdery mildew. The tobacco mosaic virus strikes a wide variety of plants - tomatoes, cucumbers, ornamentals, lettuce, beets, peppers and even weeds.
If, after your best efforts to avoid problems, you suspect a plant disease has appeared, your first mandate is to identify the problem. Look for as many characteristics as you can, such as leaf discoloring (yellow, brown, mottling), growth changes (stunting), leaf changes (wilting, curling, malformed, smaller), spots of dead leaf tissue, unevenly ripened or fewer fruit, etc. This will be needed to positively identify the problem and determine the next step. There may be possible treatments (e.g., spraying, a soil drench, change in watering practices, trimming off affected parts) or again, the best advice may be to remove and destroy it.
Viruses can only be confirmed by a plant pathology laboratory and are incurable. Your best bet if you suspect a virus is to pull it up and destroy it - now. It may be sad but a better resolution than allowing it to spread and potentially replicate for a long, long time in your garden.
You must use a reputable source to find reliable information to identify a plant disease and possible treatments. Tomatoes, as an example, are sometimes stricken by diseases that are harder to identify because they share so many characteristics. A .edu source such as the University of Minnesota, Cornell University, or the University of Connecticut would be excellent resources. The U of M’s Yard and Garden Line has a ”Plant Diseases” section where you can click on “What’s wrong with my plant?” (https://extension.umn.edu/yard-and-garden#solve) There you will find photos of common problems to guide you.
Diseases can really put a damper on your experience. However, with good garden management and reliable information, you can maintain a healthy garden and avoid many virus and disease problems in the first place.