Why do vegetables continue to die no matter how many times I replant? There are soil borne plant diseases that can survive in the garden, either on plant debris or in the ground itself. Once infected, the soil can remain so year after year. Some of these diseases have a wide host range, while others attack either one particular species only or may attack members within a particular plant family. Planting the same crop year after year in the same location will allow for the pathogen to build up to high levels.
One of the best control measures for the soil borne diseases is crop rotation. This will allow time for the plant debris to decay and the associated pathogens to die out. The time required for this to happen varies. Some diseases survive up to 10 years or more, but most recommendations suggest periods of one to four years before returning to the original crop.
Keep in mind that a fungus that attacks, for example, the roots of lettuce will usually not attack beans. If tomatoes are being grown one year, do not come back and plant tomatoes again, or potatoes, peppers and eggplant for that matter – they are all in the same family. This is a good practice even if no diseases are detected.
Another important step in disease prevention is to take advantage of any resistant varieties that may be available. For vegetables, check with seed catalogs to see if they have varieties that are resistant to a specific disease. The same is true with fruit trees since certain varieties are more resistant to diseases.
Note: resistant (or tolerant) does not mean immune. A resistant plant will be less affected by the disease and show fewer symptoms. A plant that is immune will not be affected by the disease. Unfortunately, some resistant varieties may lack certain desirable characteristics in the plant such as flavor or high production. If you can find resistant varieties, the battle against disease is half over.
Ask the Garden Guy, Science Based Answers to Garden Questions, is an excellent resource book for gardeners. Some popular topics include Slugs and Snails, Organic Pesticides, Why Vegetable Seeds Do Not Germinate, What’s So Hot about Manure? Mushrooms in the Lawn. Purchase by clicking on the image of the book above.