Fruit IPM Fruit IPM
Mark Longstroth,
District Extension Educator for Horticulture
and Marketing
Here in Michigan fruit growers are beginning to understand that many of the very effective materials that they have used for 5 to 10 years no longer work as well as they used to work. It is apparent that there is resistance in apple scab, cherry leaf spot, black rot of grapes and sevreal othe fruit diseases to materials that seemed to be wonder drugs 10 years ago.
Growers and pest management consultants need to be concerned about fungicide and bactericide resistance and should incorporate strategies to delay the development of resistance in plant pathogenic diseases. In Michigan, we have documented examples of materials which are no longer effective because of resistance in the pest population. Dodine (Syllit), benomyl (Benlate), and thiophenate-methyl (Topsin-M) are no longer recommended for apple scab; streptomycin resistance has appeared in fire blight; and copper resistance is common in bacterial canker.
To prevent or delay resistance to fungicides and bactericides we can employ several strategies:
Unfortunately economics do not always make this easy. Another problem is that some materials are so effective they are our primary or only choice in our disease control program.
The sterol inhibiting (SI) fungicides (Nova, Rubigan, Bayleton, Indar, Orbit, Procure, and Elite) are at risk for resistance because they are so widely used. They have been effective and they can be applied after a rain and still prevent disease development. This "back-action" makes the SI's the backbone of most disease control programs in Michigan and their loss would result in substantial losses to the fruit industry. Therefore it is very important to use other materials with different modes of action whenever they are effective. In grapes, the SI fungicides have been used extensively for over six years to control black rot and powdery mildew. In Michigan, we see increasing tolerance in black rot to these materials and powdery mildew resistance has been found in New York and California. Before bloom grape growers use captan or EBDC-type fungicides, materials which are protectants, then after bloom they use Nova or Bayleton, which were very effective six years ago, but they are not as effective now. In apples, Nova and Rubigan have been used to control scab for some time and these materials are not as effective as when they were first available. The first documented case of SI resistance to apple scab in the US arose in Michigan (Fig. 1). I have seen growers switch to complete protectant programs and say they have less scab than when they used SI materials alone. I am sure part of this is because they know that they need to be covered all the time. I suggest to my growers that they use combination sprays combining a SI with a protectant, especially before and after bloom.
Combination sprays have the advantage of assaulting the pest in two different ways at the same time. It is much harder to develop resistance to two different materials at the same time than it is to develop resistance to one. More and more we see the chemical companies releasing reformulations of their products where the material is a mixture of two materials with different modes of action. This is an effort by the manufacturer to extend the useful life of the product.
Alternating sprays using materials with different modes of action is an another way of delaying resistance. When alternating materials, I stress that the fungus should encounter a completely different mode of action in the spring than in the previous summer and fall. This will decimate resistant individuals at the beginning of the year. When you apply several sprays of a material you select for individuals that are resistant to that material. That is the population you want to destroy with a completely different mode of action.
Many growers think that alternating materials means just slipping in a spray with a different mode of action then returning to their old standby. It is probably much more effective to apply 2 or 3 sprays of one material and then switch to another. You would apply 2 or 3 sprays of one at-risk material and as the resistant population begins to increase you would switch to another material to which it has no resistance, effectively killing the resistant individuals. This will maintain a materials usefulness for much longer.
Sometimes, we worry that while we plan our sprays to delay the development of resistance, our neighbors do not. By using the same tactics to delay resistance development we suppress the movement of resistant genes into our pest population. Your neighbor, however, will no longer be able to use that material to control the pest.
If you used one material or several materials with the same mode of action on your crop last year, I urge you to use another with a different mode of action this spring. Kill as many resistant individuals as you can before you go back to last year's standby. Growers can use the fungicide efficacy tables in the Michigan Fruit Management Guide E-154 to determine which materials have the same mode of action. Materials in the same family have the same mode of action and should not be used as alternatives or in tank mixes together. There is a talbe grouping fungicides by mode of action in the fungicide section of E-154.
It is very important that sprayers be effectively calibrated. Uneven or poor coverage does become apparent when we are using a material which is highly effective in small dosages. But as resistance or tolerance increases poor coverage will mean poor control because not all the leaves or fruit received an effective dose. Timely sprays are also important. It is easier to prevent a disease from getting started than it is to stop its spread. Applying materials late to control a disease which is already present and spreading in your fields increases the opportunity for resistance increases dramatically because of the increased amount of inoculum in your plantings and the variability present in that population.
This strategies are important for isolated growers. They are even more important in those areas of the state where we large areas of a single crop growing in a region. Once resistance gets started it can move through the entire region. It is essential to adopt strategies that delay resistance even if it means increasing the cost of pest management in the short run to save alternatives for future use. Expecting the agri-chemical companies to continue to develop new effective chemistries in minor crops is an optimistic approach with potentially serious long-term consequences.
Fig. 1. Reduced sensitivity to SI fungicides of a Venturia inaequalis population (closed bars) from a commercial apple orchard in Michigan. Open bars indicate the sensitivity of a scab population that had not been treated with SI fungicides. Data are from Phytopathology (1997) 87:184-190.
