There seems to be some confusion about what is a proper crop rotation to manage diseases caused both by pathogens that survive a few years in soil and pathogens that survive in soil longer than anyone can measure. Here are some rotations I have heard about that are not proper crop rotations.
1. A susceptible crop in year 1 and a different susceptible crop in year 2. Example: Pepper followed by tomato or tomato followed by pepper in a field infested with Phytophthora capsici, the water mold that causes Phytophthora blight. Simply switching between two susceptible crops is not proper crop rotation.
2. A susceptible crop in the spring and another crop in the same plant family in the fall. Example: Watermelon in the spring and cucumber in the fall on the same plastic. There are too many potential disease problems with this crop sequence to even mention all of them. Gummy stem blight, root knot, and Phytophthora blight are among the top three.
3. Multiple plantings of a short-season crop in the same field in the same year, then rotating the following year. Example: pickling cucumbers. The same comments made in #2 apply here. Root knot nematodes on an early summer crop will infect a crop planted later in summer.
4. A susceptible crop in the fall and a different susceptible crop in the spring in the same field. Although I don’t have a specific example this time, the short break over winter is not long enough to reduce pathogen levels.
In general, a proper crop rotation is a sequence of crops that are in DIFFERENT plant families. Although not every disease affects all plants in the same family—e.g. early blight on tomato and potato but not on eggplant or pepper—enough diseases do affect closely related vegetables that it is best to avoid planting them too often right before or after each other.
Dr. Tony Keinath with a word on crop rotation. “To keep the soil on your farm productive over the long term, do not replant the same vegetable, or a related crop, in the same field “too often.” How often is “too” often depends on the crop and the pathogens present in the soil. Almost always, “too often” is less than 12 months between disking the old crop and planting the new crop. A general rule of thumb is 24 months between related crops, and in some cases, 36 months (3 years) is needed.
The main risk in replanting “too often” is building up root pathogens that survive in soil for years. Even in the heat, diseased roots and stems take several months to decay enough so they are not a source of pathogens. Thicker tissue, like the crowns of cantaloupe, can take 2 years to decay.
Another risk is foliar diseases that start on volunteers from the previous crop. The pathogens may be in or on some of the seed that sprouts, for example black rot on leafy brassica greens or gummy stem blight on cantaloupe and watermelon. A small number of infected volunteers means the disease has a head start right at the beginning of the crop.
Controls for soilborne pathogens (fungi, water molds, and nematodes) are limited.
Many vegetable crops have no resistance to these pathogens.
Fungicides do not penetrate soil well, or they are quickly inactivated.
Fumigants have many restrictions that require time-consuming record keeping and air monitoring.
Root-knot nematodes are a special problem, because they form galls on many vegetables and some field crops (cotton, for example) grown in rotation with vegetables. Summer cover crops of sunn hemp can lower nematode numbers. See https://edis.ifas.ufl.edu/in892 for a list of other warm season cover crops that help manage nematodes.
Without crop rotation, more fungicide sprays will be needed, which raises the risk of fungicide resistance. Fungicides and fumigants are not a substitute for good crop rotation.”
Justin Ballew reports, “Many areas got a little more rain last week and we had a break from the heat over the weekend. We have a lot of the fall crop planted now including squash, zucchini, tomatoes, and brassicas. So far everything is mostly growing well. We are seeing bacterial wilt develop in some of the fall-planted tomato fields. Bacterial wilt loves hot soil temperatures, which is typical this time of year. Be sure to follow a proper crop rotation plan (at least 3 years) to help manage bacterial wilt buildup in fields. Since the heavy rain we got a few weeks ago, we’ve also seen plenty of bacterial spot in what’s left of the spring tomatoes.”
Bruce McLean reports, “Most summer vegetable crops are starting to wind down. Peas, okra, watermelon, cantaloupe still harvesting at some volume. Late summer/fall squash, cucumbers and tomato harvest are a couple weeks away. Fresh market muscadines are being harvested now. Juice and wine muscadines are getting close – maybe 7-14 days away, depending upon location. In most juice and wine muscadine vineyards, Carlos is around 40% colored (ripe), Noble is around 60%, and Doreen is around 25%. Grape root borer (GRB) flight is still occurring, with moderately high moth counts in traps.”
Tony Melton reports, “‘Another Crop Bites the Dust’ from spreader/stickers. I have seen too many farmers sing this sad song. Be careful and know what you are doing when adding a sticker/spreader when spraying vegetables. Short season, tender vegetable crops will burn very easy in our heat and do not have time to come back like long season row-crops. Our state’s second major watermelon/cantaloupe season is in full season in Chesterfield County. Harvest is in full swing and will continue until frost. Curculio sprays are beginning to be applied to the fall pea crop. Looks like pickle harvest will continue until frost.”