MOFGA's Pest Report - September 9, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist
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 thing you can do is to give them a dependable food source year after year. The problem will likely get worse and worse. The most effective way to avoid this is by using crop rotation. 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.
The 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 moving your host crops will not make much 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 decide to put your beans or potatoes in will not make much of a difference. In contrast, the Colorado potato beetle hibernates during the winter and essentially 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 or disease affect? If the pest is very specific in what it lives on 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 the 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, in addition to a tool for pest management, is very important for managing nutrients in the soil and for controlling weeds.
Sanitation (Late Blight): Late blight is popping up in lots more locations now. It is not the epidemic proportion of gardens and farms that it was in 2009, but with the recent wet, cloudy and windy weather it has spread quite a bit. Late blight is one of those diseases for which sanitation, especially at the community and state level, is more important than crop rotation. The pathogen that causes the disease does not persist in the soil. In fact, here in New England it does not persist any where except in living host tissue. So, avoiding it next year is not dependent so much on which field you grow tomatoes or potatoes next year as much as it is on how you handle (and other folks in the region handle) the diseased tissue this year. Any of the plant parts that die will not carry the disease forward to next year, e.g., it is not able to persist on any part of a tomato plant unless you somehow keep that plant alive through the winter. And, it only persists on the potato tuber of the potato plant. The most common way it gets from one season to the next is by people planting diseased potatoes in the spring, or not scouting for and removing any potato plants that arose from cull potatoes laying around. Late blight is a great example of a disease that can be managed very well by simply having good sanitation.