MSU Extension

The spring of 2012 is one for the record books. Early high temperatures moved growth and development along at alarming rates. Then, several freeze events and colder than normal temperatures have led to a reduced fruit set for the 2012 apple crop. Crop guesstimates are still to be determined, but in general, it looks like the apple crop may be less than 10 percent in many Michigan locations. There is much variation in fruit set due to location and site considerations, differences in varieties, and quality of pollination and fertilization of flowers.

For those growers with crop insurance, the guidelines of your policy commonly state that you have to maintain the trees in a normal fashion in order to qualify for claim payments. With reduced or no crop on the tree, several insects and diseases may be ruled out of your spray programs. It is difficult to justify costly applications to trees with little or no crop, but there are some pests that you need to be aware of to prevent problems for the 2013 season.

Fire blight. The southern half of the state is past the blossom period where blossom blight is of greatest concern. However, trauma blight situations that may occur with high winds and hail can still cause devastation to orchards. Trauma blight situations should still be managed with applications of streptomycin or copper. Copper applications can help to slow down fire blight, but will be somewhat harsh on tree growth. Streptomycin is the best antibiotic material we currently have labeled for fire blight in a post-trauma blight situation where resistance is not an issue.

Growers should patrol orchards to remove active fire blight cankers promptly. Apogee applications post bloom is very useful for managing fire blight by suppressing excess terminal growth on susceptible varieties. Apogee does take advanced planning to judge the treatment amounts and timing. Apogee treatment takes 10 or more days to see the effects. Apogee on high value blocks, highly susceptible varieties and where fire blight was a problem last year should be applied and at higher seasonal rates due to the very light crop.

Apple scab. Hopefully you are staying ahead of primary scab this year. If you have scab in blocks with little crop it would be best to get the lesions under control before you reduce or eliminate fungicide application. Blocks with heavy scab in them prior to June 1 could defoliate early, leading to reduced winter hardiness and a high potential inoculum level for 2013. Once primary scab season is over, you can reduce further control measures if you had primary scab under control. Use of lime sulfur and even copper are lower-cost alternatives to scab control. Multiple applications of lime sulfur or copper can be harsh on tender tissue.

Powdery mildew. Just as with apple scab, powdery mildew left uncontrolled in highly susceptible varieties can lead to reduced winter hardiness. Again, as with apple scab, most commercial blocks have had some mildewcides in their programs already this year, so mildew might not be of concern in most blocks with no crop. If you have running mildew right now, it could reduce winter hardiness of buds and lead to a higher inoculum potential for the 2013 season.

Plum curculio. Under light fruit load conditions, plum curculio will compete heavily for the fruit that is present. Unprotected this can result in a much higher percent damage level than normal. Most of this fruit will drop, but the larvae that emerge could be the source of next year’s “resident” population. Growers with substantial plum curculio pressure in the past may want to treat, however, overall this pest is not of real concern for this year due to the near zero fruit on the trees.

Potato leafhopper. Potato leafhoppers are normally controlled when broad-spectrum insecticide programs are used to control primary pests like plum curculio, codling moth and oriental fruit moth. If you are reducing or eliminating insecticides for these key pests because of little or no crop, potato leafhoppers should not be ignored, but rather monitored closely – especially in non-bearing trees that need to fill their space. Potato leafhoppers typically first arrive in late May with southerly-based weather fronts, but in this strange weather year, they are present now. Those adults lay eggs, which hatch and begin feeding (phloem feeders) on foliage of actively growing terminals in mid-June, often reaching high populations by early July. The resulting damage appears as necrotic, cupped-leaf margins and can stunt growth significantly. Control will be particularly important in young blocks, especially highly sensitive European plums, that still have space to fill. This pest can, some years, do considerable damage, but is of less importance to control.

Obliquebanded leafroller. The obliquebanded leafroller is largely a foliage feeder, but can do significant damage to fruit. Fruit damage is most common under conditions of heavy fruit set where full clusters and adjacent foliage prevent adequate penetration of targeted insecticides. Thus, light fruit-load conditions like this year should reduce the risk of obliquebanded leafroller damage compared to normal years. Specific control for obliquebanded leafrollers in blocks with no harvestable crop is probably not warranted. The use of Apogee will help to set terminal buds and drive obliquebanded leafrollers to other hosts plants.

Codling moth. This pest should be monitored the entire year. If you have a few fruits on the trees – perhaps as few as 10 or 20 fruits on a dwarf tree – codling moth will easily infest these fruits in their first generation if you eliminate cover sprays for codling moth. This can lead to very high codling moth numbers and increases the potential damage for the 2013 season. Growers may want to take advantage of the light crop and late emergence of codling moth from overwintering sites due to cool weather by delaying the first codling moth spray by 100 degree days or so. For example, targeting egg hatch at 350 degree days instead of 250 degree days. The delayed application will better target peak activity and perhaps provide sufficient control with a single application. This is especially the case where mating disruption has already been applied.

A good codling moth management approach with the very light crop would be to apply hand-applied mating disruption at a low rate of 200 dispensers per acre. If you have orchards with a crop on them, you should be very aware of any nearby orchards that may be on a reduced insecticide program because of no crop. Codling moth will move from an orchard with little fruit to a neighboring orchard with fruit. This can happen with the first generation, but can be more of a concern for the second generation.

Oriental fruit moth. This insect can be a significant problem in apples, especially in peach-growing regions. Larvae can bore into ends of new growing terminals or into fruits. If insecticide cover sprays are eliminated from apple blocks, oriental fruit moth and other insects will build in number, likely increasing pest pressure somewhat the following year. Oriental fruit moth can be a problem in terminals especially in young blocks. It should be closely monitored for its activity in 2012.

European red mites. Left uncontrolled, European red mites can reduce photosynthesis and overwintering carbohydrate reserves. These reserves provide the tree with its winter hardiness, as well as help set the next year’s crop. European red mites can cause severe bronzing, but if this occurs in a year without a crop, the damage will not be as severe, due to the lack of competition for the carbohydrates from fruits. In fact, if certain broad-spectrum insecticides are left out of an orchard system (codling moth, for example), then mite predators will have a chance to build their populations to help curb the European red mites. European red mites are of most concern on young plantings.

Other things to think about…

Benefits of beneficials. One possible benefit of reducing broad-spectrum insecticide sprays would be a potential increase in biological control organisms such as beneficial insects, which could be helpful for your orchard system for the future.

Return bloom for 2013. Next year will most likely have a tremendous return bloom. The crop potential could be huge for Michigan. With little crop, the vegetative growth should be at a maximum for 2012 – leading to extra pruning for the dormant season. A strong dormant pruning program will help regulate the 2013 crop. Apogee applications will help reduce terminal growth, but much of the state is out of the window for good growth control with Apogee for this current season.

Eliminating fruit. If you have less than 30 percent of a crop, you might want to consider eliminating fruit completely from the trees. You can limit the infestation from the apple insects, codling moth and apple maggot by eliminating the fruits on the trees. Choose warm temperatures (70 to 80 degrees) to apply thinners. Chemical fruit removal can be done with the highest labeled rates of spray thinners, such as NAA or Sevin XLR. Chemical thinners should be applied as soon as the flower petals are 80 percent fallen (not too soon in bloom or you can harm pollinators). A second application 10 to 14 days later may be needed to remove more fruit. Even with two applications of chemical thinners, there may be some fruit remaining that may need to be removed by hand. Suggested fruit removal program: 20 PPM NAA (8 oz. NAA in 100 gallons of water) PLUS 1 quart Sevin XLR.

Large fruits. Fruit size will most likely be large on trees with a light to moderate crop set. Large fruits have some potential inherent problems such as bitter pit, water core, and cracking, which can cause storage and marketing problems. Calcium sprays can help and might be justified in certain higher value varieties that commonly have problems such as bitter pit. Reducing nitrogen applications is also suggested to reduce the potential for bitter bit.

Scarred fruits. There is likely to be a lot of surface damage on apples this year due to the cold weather during and after bloom. Growers should carefully evaluate crop quality – if it is poor, eliminating the fruit and using a reduced spray schedule could be considered. However, with the whole state apple crop affected this year, processing and juice apples might bring some income.

In conclusion

Be sure of your crop situation before you decide to eliminate cover sprays entirely from an apple block. Apple fruit set can fool the eye sometimes – one week it may look like a total loss, and the next week the fruit will start to show up more readily. Also, if you have crop insurance, be sure to check with your insurance representative the details that they may require of your pest management program so that you are not disqualified in any way.