CROP ROTATION: Most of the disease, insect and weed problems you will have this season have already occurred and now in addition to maintaining your control of these you need to start thinking about next season. For many pests the severity of the problem next year will be the result of what you do this year. For insects and diseases the worst practice you can do is have a dependable food source available for them year after year. The problem will likely get worse and worse. The most effective practice to avoid this is crop rotation. That is why the National Organic Program Rule requires crop rotation where it is appropriate. Now is the time to decide where crop rotation will work to disrupt the dependable food source your insects and disease pathogens hope to find.
Effectiveness of crop rotation depends on the life history and biology of the particular pest. The three characteristics that play the biggest role are:
- Dispersal ability- How well and far can the pest move? If the pest can only move short distances then by rotating your crops to a new field you can leave them behind unable to reach food before they starve. On the other hand, if the pest typically moves long distances then if you put your host crops here or there it will not make a bit of difference. For example, the potato leaf hopper comes here all the way from the Gulf of Mexico region and so chances are that which field you put you beans or potatoes in will not make a difference. In contrast, the Colorado potato beetle hibernates during the winter and essential can only walk to potatoes when it wakes up in the spring. You do not have to move potatoes too far from last year's field to significantly reduce the numbers that find the new planting.
- Host Specificity- How many different kinds of plants does the pest feed on? If the pest is very specific in what it eats it is easier to choose an alternative crop to plant where the pest is overwintering. On the other hand, if the pest feeds on many crops (or weeds) then it is difficult to avoid a food source for it and it will probably be waiting for you no matter where you move your crop or how long you wait to plant it again. For example, the tarnished plant bug, which is a major pest on lettuce (browning of the midrib), strawberries (catfacing), eggplant (feeds on tiny buds and they drop) and broccoli (brown beads in head), also feeds on about 300 weeds and so really does not depend on your crops to survive. In contrast, the species that causes Septoria leaf spot on tomato can only survive on plants in the tomato family (tomato, eggplant, potato, petunia,
black nightshade, etc) so it is not that hard to plan a successful crop rotation to leave it without a host to feed on.
- Persistance- How long can the pest survive without a host? This is a critical question because some diseases can persist in some sort of resting spore for very long periods of time waiting for a host to appear again. Most insects need food every season or they starve and many diseases can only survive a year or two without a host. For example, the pathogen that causes late blight of potato and tomato can only survive on living tissue and so in New England it really only makes it from season to season on potato tubers. Proper handling of culls is key to managing this disease. In contrast, the pathogen that causes white mold of beans, carrots, tomato, lettuce, etc. can form resting bodies called sclerotia that are able to just sit in the soil for many years without any host.
So, now is the time to look around the farm and see what you have for pests and learn which ones can be managed with crop rotation and then make plans. Crop rotation does not work for pest management in gardens because almost any pest can move from one side of a garden to another, especially when the gardener pushes the tiller back and forth. That does not mean that gardeners should not rotate. Crop rotation, in addition to a tool for pest management, is very important for managing nutrients in the soil and for controlling weeds, and this is true for both farmers and gardeners.
Rotations used to control weeds include rotations with other crops as well as rotation with fallow ground or ground put into cover crops instead of cash crops. Fallow periods allow the grower to periodically till the ground which kills growing weeds and stimulates weed seeds to germinate that later can be killed by the next tilling. This practice is of course bad for the soil structure, but in situations may be the best approach for cleaning up a field before going back to good soil building practices. In situations where weeds are totally out of control the fallow period may be long and include lots of soil-damaging tillage. But, in later, once the weeds are under control, the fallow period can be shortened and most of the rotation can be with cover crops instead of bare ground and tillage.
Basic rotation plans to control weeds, in addition to rotating with fallow periods, include:
- Rotating vegetable crops that are hard to cultivate and keep weed clean with those that are easy
- Growing short season crops that allow half a season for cover crops
- Alternate early season crops with late season crops to allow attacking spring weeds one year and fall weeds the next
- Take land out of production and grow a series of short season cover crops such as oats, then buckwheat, then oats for the fall
Rotations used to manage soil fertility include planting fall cover crops to avoid loss of the many nutrients left after crop harvest, rotations of heavy feeding crops with light feeders, rotating crops that tap shallow soil with those that tap deep soil and rotating nitrogen fixing green manures with cash crops. Using crop rotation with nitrogen fixing legume green manures is a prime way that farms can reduce their environmental impact. Not only are nitrogen rich fertilizers environmentally costly to produce they also may indirectly lead to excessive phosphorus in the soil. Typically, farmers and gardeners determine how much soil amendment to add based on the nitrogen analysis of the material and many materials typically used to add nitrogen (ex. Manures, organic waste, etc) also add phosphorus. After years of this practice it may result in excessive phosphorus in the soil that may end up as pollution in surface water. Using nitrogen fixing crops in a rotation adds nitrogen to the soil with no other nutrients.
Whether you are a farmer or gardener it is time to plan your rotation now. For more information on using green manures in crop rotations, including fall cover crops, see MOFGA Fact Sheet #10 Using Green Manures.
PUMPKIN AND WINTER SQUASH HARVEST AND STORAGE (reprinted from Umass Veg. Notes): Although there are many fields with immature fruit, pumpkins in some fields are orange. Sugar pumpkins, especially, are ready early. Butternut in some fields is showing the dull, dry skin that characterizes mature fruit. If the current warm sunny days continue [not warm here in Maine anymore, is it?], more and more pumpkin fruit will color up in the next several weeks. Fruit sitting in the field faces a daunting list of diseases and insects - not to mention possible passing hurricanes -- that could threaten fruit quality. Early harvest and careful storage is preferable to leaving fruit in the field. This is especially true if you know that your pumpkins or squash are in fields that are infected with Phytophthroa blight.
Pumpkins may need to be held for several weeks before they can be marketed. There can be extra work involved in bringing fruit in early, especially for growers who normally have pick-your-own harvest, but we recommend that growers harvest as soon as crops are mature and store under proper conditions, if it is feasible. Attention to curing and handling will go a long way toward improving the life of winter squash and pumpkin fruit. What about pumpkin stems, i.e., handles? In some cases, it's the handle that sells the pumpkin. Pumpkins are not marketable if the handle is broken off or dried up. Ideally, if the timing is right, pumpkins would be cut one to two weeks prior to marketing. However, if they are harvested now they may sit much longer before being sold. The discussion of how early to cut handles is an old one with many different opinions. One view is that it is advisable to cut the handles from the vine to save them from advancing powdery mildew and reduce shrinkage. Whether or not handles shrink and shrivel after cutting is affected by plant stress, genetics (variety), moisture and temperature conditions, and disease. There are many diseases that can affect handles, including Plectosporium, Fusairum, Black Rot, and Alternaria. Again, proper curing and storage conditions are key. Ideally, pumpkins should be harvested when fully mature, with a deep orange color and hardened rind. However, as long as pumpkins have started to turn color, they will ripen off the vine if held under the proper conditions. While not ideal, this may be preferable to leaving them in the field if conditions are not favorable. If necessary, pumpkins can be ripened in a well-ventilated barn or greenhouse. The best temperatures for ripening are in the seventies to low eighties. Night temperatures should not drop below the sixties. Even if pumpkins are ripe, a period of curing can improve storage life. The curing period should be about 10 days. During this process, the fruit skin hardens, wounds heal and immature fruit ripens - all of which prolongs the storage life.
Pumpkins should be stored in a cool, dry place. Ideal temperatures are between 50° and 60° F and relative humidity of 50 - 70%. Higher humidity allows condensation on the fruit with risk of disease, and lower humidity can cause dehydration. Higher temperatures increase respiration and can cause weight loss. Temperatures lower than 50° F cause chilling injury (see squash, below). In a greenhouse, temperature can be managed with ventilation on sunny days. Unless it is quite cool, heat is not likely to be needed if the house is closed up at night. Often it is not feasible to harvest pumpkins early and store them until they can be marketed, and so they must be 'stored' in the
field. If vines and fruit are healthy, storage in the field can be successful for a few weeks. If the vines die back, damage to the fruit from sun, disease and insects is more likely. In any case, it is important to scout for insects feeding on the fruit and handles, which may include squash bug nymphs or adults, or striped cucumber beetle. Control them if damage is evident. In fields that have a history of Phytophthora blight, Fusarium fruit rot, or black rot, field storage may increase the incidence of these problems, particularly if we have a period of wet weather or a major storm while fruit is sitting in the field. This has been one of the causes of significant losses in recent years, and one reason that we recommend bringing fruit in as soon as it is mature.
Winter squash is also maturing in some fields. Fruit that are free from disease and haven't been subject to much chilling (below 50°F ) should be selected for long-term storage. Fruit from fields where Phythophthora is present are not the best choice for storage. Storage life depends on the condition of the crop when it comes in and your ability to provide careful handling and a proper storage environment. All fruit placed in storage should be free of disease, decay, insects, and unhealed wounds. When harvesting squash and pumpkins, it is important to handle the fruit with care to avoid bruising or cutting the skin. Despite its tough appearance, squash and pumpkin fruit are easily damaged. The rind is the fruit's only source of protection. Once that rind is bruised or punctured, decay organisms will invade and quickly break it down. Place fruit gently in containers and move bins on pallets. Use gloves to protect both the fruit and the workers. Removal of the stem from squash (butternut, Hubbard, etc.) will also decrease the amount of fruit spoilage because the stems frequently puncture adjacent fruit, facilitating infection. A period of curing after harvest can help extend storage life of squash. This may be done in windrows in the field -- especially with a series of warm, dry days -- or by placing squash in a warm dry atmosphere (70-80°F) with good air circulation, such as a greenhouse, for up to two weeks. This pre-storage treatment permits rapid drying of the outer cell layers, and when combined with a dry atmosphere for storage inhibits infections that can take place at this time. Any clean cuts during harvest a likely to heal over and are no longer a source for injury or infection.
Take care to avoid subjecting squash to chilling injury. Chilling hours accumulate when squash or pumpkin is exposed to temperatures below 50°F in the field or in storage. Injury increases as temperature decreases and/or length of chilling time increases. Chilling injury is of particular concern with squash intended for storage because it increases the likelihood of breakdown. If squash has been exposed to chilling injury it should be marketed first and not selected for long-term storage. Remove squash from the field if temperatures likely to drop below fifty degrees for any length of time.
After curing, move squash or pumpkins to a dry, well ventilated storage area. Pressure bruises can also reduce storage life, so avoid rough handling, tight packing, or piling fruit too high. Fruit temperature should be kept as close to the temperature of the air as possible to avoid condensation, which can lead to rot. Ideally, the storage environment should be kept at 55-60°F with a relative humidity of 50-70%. Lower relative humidity increases water loss, resulting in reduced weight, and if excessive, shriveling of fruit. High relative humidity provides a favorable environment for fungal and bacterial decay organisms. Under ideal conditions, disease-free pumpkins should have a storage life of 8-12 weeks and butternut squash up to three or four months. Even if it is difficult to provide the ideal conditions, storage in a shady, dry location, with fruit off the ground or the floor, is preferable to leaving fruit out in the field. As you plan for storage and marketing, keep in mind that the market for pumpkins seems to get earlier every year. Fall decorative displays include pumpkins, and those displays begin showing up as Labor Day approaches. One of the best solutions to early-maturing pumpkins may be finding an early market. With so many late-planted field this year, early pumpkins are likely to be in demand. --R. Hazzard; many thanks to the following sources: J. Howell, A. Carter, and Robert Wick. University of Massachusetts; Dale Riggs & Robert Rouse, Pumpkin Production Guide, NRAES; Maurice Ogutu, University of Illinois Extension; and Liz Maynard, Purdue University
ALTERNARIA DISEASES OF BRASSICAS (modified from a report in Umass Vegetable Notes by Beth Dicklow and Rob Wick): Three species of Alternaria cause serious damage to brassicas: Alternaria brassicicola, A. brassicae, and A. raphani. Alternaria brassicicola and A. brassicae infect broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kohlrabi, kale, rutabaga, and turnip. A. raphani is most often found on radish, but can infect other brassica crops. The most common symptom of Alternaria diseases is yellow, dark brown to black circular leaf spots with target like, concentric rings. Lesion centers may fall out, giving the leaf spots a shot-hole appearance. Individual spots coalesce into large necrotic areas and leaf drop can occur. Lesions can occur on petioles, stems, flowers, flower pedicels, and seed pods. Pod infection causes distortion, premature shattering, and shriveled, diseased seed that germinates poorly.
Alternaria species are simple parasites that survive saprophytically outside the host. Diseased crop debris is the primary site of survival from year to year. Resting spores (chlamydospores, microsclerotia) have been reported. The disease is favored by warm temperatures (60-78° F) and at least 12 hours of relative humidity of 90 % or more. The fungi sporulate profusely and are spread throughout fields by wind, splashing water, equipment, and workers. The main means of introduction into new areas is on infested seed.
- Buy seed certified as disease-free or treat seed with hot water. [for details on hot water seed treatment, see The Resource Guide for Organic Insect and Disease Management, page 12]
- Practice long rotations with non-brassica crops.
- Incorporate diseased plant debris into the soil.
- Eliminate cull piles.
- Control brassica weeds.
- Avoid overhead irrigation during head development.
- Keep seedbeds disease-free to prevent the spread of disease and locate seedbeds so as to avoid wind-borne inoculum.
- Control of Alternaria leaf spot on cabbage heads in the field is necessary for long-term storage.
Copper compounds are labeled for Alternaria in brassica crops but recent studies have shown poor results.