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Disease - 218-The Influence of Pruning Methods on Disease and Insect Control

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Fact Sheet No.218, UMaine Extension No. 2238

Prepared by Tom DeGomez, Extension Blueberry Specialist, David H. Lambert, Assistant Professor of Pathology, H.Y. Forsythe, Jr., Professor of Entomology, and Judith A. Collins, Research Associate. The University of Maine, Orono, ME 04469.  January 1990.

Introduction

There are several effective methods to prune lowbush blueberries:  burning with oil or propane-fired burners, straw burning, free burning (generally less effective), and flail or rotary type mowers.  Each has both merits and drawbacks in keeping insects and diseases under control. In recent years, growers have observed an increase in insects and disease.  Increases in pests may be due to changes in cultural practices, the biggest of which have been in the areas of pruning and weed control.

In many instances, pruning can be accomplished with flail type mowers, rather than more costly traditional methods of oil or straw burning.  When fields are mowed, they need to be continually and closely examined for disease and insect infestations.  Expanded monitoring by trained scouts has led to increased awareness of potential problems.

Pruning with mowers is a fairly new management practice.  In the early 1980s, research at the University of Maine demonstrated that flail-mowing, if done properly, is as effective as burning.  Pruning with mowers costs less, uses less non-renewable fuel (than oil burning) and eventually increases the organic matter on the soil surface.

However, pruning with fire offers advantages that mowing does not.  The heat of the fire will not only kill the stem, but may also reduce the incidence of insects, diseases and weeds. Insects, including flea beetle, spanworm and sawfly over-wintering on or near the soil surface, may be killed by the fire’s heat.  Fire pruning may also destroy overwintering fungal organisms, such as the Mummyberry fungus, and certain fruit rotting organisms, which live on dead leaves and stems on the soil surface.  Traditionally, lowbush blueberry growers have used fire to prune. As a result, diseases and most pest insects have not been a major problem.

Diseases

Mummyberry (Monilinia blight) disease has been the most destructive lowbush blueberry disease in Maine in the past five years.  It has been an economic threat to many fields that have been flail-mowed for multiple cycles.

Weather and field conditions have also affected the incidence of diseases.  Wet weather during bud break or high water tables can contribute to outbreaks of Mummyberry spore production and infection in burned or flail-mowed fields.  Dry weather, on the other hand, promotes spore release from infected tissues.  Botrytis blight is most severe during mid-to late-bloom, when cool, rainy weather follows a frost.  Other diseases, such as red-leaf and powdery mildew, have been prevalent but have not been linked to crop losses or mowing practices.

Recent studies at the University of Maine Agricultural Experiment Station show differences in disease incidence because of pruning practices.  Five fields with side-by-side comparisons of pruning treatments were closely monitored for Mummyberry, powdery mildew and red-leaf diseases.  In areas that were flail-mowed, there was no greater incidence of red-leaf or powdery mildew than in adjacent areas, which had always been burned.  Mummyberry dsease increased ninety-fold in the first field, which had been flail-mowed for six cycles and decreased six-fold in the second field after two cycles of burning after mowing.  These results indicate that Mummyberry disease does increase over time with flail-mowing and that a short return to burning will reduce the disease but will not eradicate it.

In 1989, fruit was harvested from the same test fields.  The incidence of fruit rot diseases was greatest on berries harvested in the flail mowed fields.  The incidence of Botrytis was not affected by pruning method. (Botrytis spores tend to be everywhere and have not been reported to build up due to pruning method.).  The fruit rots that are at higher levels in flail-mowed fields are Glomerella (anthranose) and Altenaria. Both of these diseases affect fresh and perhaps frozen fruit quality.

Growers who choose flail-mowing over burning should monitor their fields for the incidence of Mummyberry disease and take corrective measures to avoid economic losses.

Insects

Insect studies, conducted from 1985 through 1989 of flail-mowed fields and burned fields have shown some species of insects to have higher populations in mowed fields.  Economic damage by spanworm larvae and, to a lesser degree, by the flea beetle has been observed on flail-mowed fields.  These insects have also been found in “lightly” burned or “scorched” fields.

Spanworm larvae have been found in consistently greater numbers in flail-mowed fields than in burned fields.  Commercial growers have experienced extensive outbreaks since 1987 on fields that have been flail-mowed for multiple cycles.

Flea beetle has been a problem in recent years for some commercial growers, as well. Outbreaks have occurred primarily in “scorched” fields, where burning was apparently not sufficient to kill overwintering stages of the insect.

The spanworm and flea beetle can seriously threaten blueberry production if populations become high.  Both these insects feed on leaves and fruit buds, and are difficult to control due to their presence at pollination.  Sprays must be timed to avoid killing pollinators. Flea beetle populations on previously infested fields dropped dramatically in 1989, possibly because of winter and spring weather conditions.

Sawfly has also appeared in consistently higher numbers on flail-mowed fields.  Sawfly has apparently not been an economic problem in the past for the commercial grower. However, flail-mowed fields should be carefully examined for this pest.

Monitoring for Pests

Whenever a management practice is changed, it will usually change some other factor in the production system.  Certainly flail-mowing has had a marked effect on the incidence of disease and insect infestations.  Producers should monitor their fields with increased intensity to detect outbreaks before they affect profits.

Action thresholds are very helpful when monitoring pests.  (Action thresholds are the point at which the number of insects or disease presence observed is great enough to indicate control may be necessary before an economic loss occurs).  It is very important to know at what point it may be necessary to control a pest.  Trying to kill every pest in a field is expensive, impossible and may lead to resistant pest populations.  It is not necessary for every pest in a field to be eradicated. However, it’s important to know when a pest becomes an economic threat.  Thresholds have not been developed for every blueberry pest, but those that are available can be helpful guidelines.

Spanworm: Monitoring should begin in the spring when buds begin to open. The best monitoring technique is a 180-degree sweep of the foliage with an insect net to capture spanworm larvae for counting; 5 to 10 larvae per 10 sweeps during daylight hours on bearing fields indicates control may be needed.  However, this action threshold may be as low as 3 to 5 larvae per 10 sweeps on vegetative fields.  Visual inspection can indicate the presence of the spanworm in a field, but it is not a reliable indicator of a high population.

Flea Beetle: The best technique for monitoring is a 180-degree sweep of the foliage with an insect net to capture flea beetles for counting.  Action thresholds have not been established for larvae or adults, but may range from 30 to 50 insects per 10 sweeps.  Begin to examine fields in early spring for larvae and from mid-June to early July for foliar-feeding adults.  Again, visual inspection can indicate the presence of flea beetles but is not a reliable indicator.

Action thresholds have not been determined for sawfly, leaf beetle, leaf tier, red­striped fireworm or strawberry rootworm.  However, these pests can also be detected with a 180-degree sweep of the foliage with an insect net.

Mummyberry Disease & Botrytis Blight: The critical time for visual inspections is during the blossom period.  Look for brown leaves and blossoms when walking in the field. Mummyberry symptoms generally appear after control is practical, thus thresholds are of little use.  The presence of this disease in a field should be used as an indicator of potential problems in the next crop.  Botrytis blight, if discovered early, can be spot treated with a fungicide.

Symptoms of Mummyberry will generally be blossom blight and browning leaves during blossom or shortly thereafter.  Leaf and fruit buds are infected at the same time; damage to the fruit will not appear until the early fruit development stage.  The browning of the leaf blade starts next to the stem and moves towards the tip of the leaf.  When spots of disease are found, often on one clone, it is a good idea to inspect surrounding clones for infection.  It is necessary to brush the stem with your arm to inspect all the leaves of the plant.  Leaf and fruit buds infected with Mummyberry often have a crust of creamy-white spores on the petiole (the small stem that attaches the leaf blade to the stem).  Later in the season, leaves killed by Mummyberry will drop, leaving short, curved, silvery “spurs” on the stem.

With Botrytis blight, there are bristle-like grayish spores on the blighted blossoms.  With heavy infestations, some leaf spots may develop, but leaves are seldom completely killed, as happens with Mummyberry.

Equipment

Disease and insect identification can be greatly enhanced using a 10x hand lens.

Insect sweep nets may be purchased from the following companies:

  • WARD’S, Natural Science Establishment, Inc., 1-800-962-2660. We recommend the “12-inch dual purpose net,” stock #10 W 0510.
  • BIO-QUIP, 1-213-322-6636. We recommend the “12-inch heavy duty aerial,” stock #73120M.

Controlling Pests with Fire

Special considerations and some trade-offs will need to be weighed if you attempt to control pests with fire.  When fire pruning it is important to control the intensity of the fire.  An overly hot fire usually means that an excess of fuel has been used, which raises the pruning cost. Also, although hot fires do a good job of pruning and controlling pests, they can destroy valuable organic matter on the soil surface.  Conversely, fires that are not intense enough will not kill stems to the ground and generally will not control pests.  To control pests with fire, ignite the litter on the soil surface, but take care to ignite the litter without destroying the permanent surface organic matter (organic pad).

There are often hidden costs to consider when implementing a control method: the destruction of organic matter, the poisoning of beneficial insects such as bees and predators, and the development of pests resistant to control methods, for instance.

For additional information on pruning and pest control in lowbush blueberry fields please refer to the following fact sheets:

  • IPM Fact Sheet No. 201, Monitoring For The Blueberry Maggot, University of Maine Cooperative Extension.
  • Wild Blueberry Fact Sheet No. 209, Insect Management Guide For Lowbush Blueberries, University of Maine Cooperative Extension.
  • Wild Blueberry Fact Sheet No. 202, Blueberry Insects 1, University of Maine Cooperative Extension.
  • Wild Blueberry Fact Sheet No. 203, Blueberry Insects 2, University of Maine Cooperative Extension.
  • Wild Blueberry Fact Sheet No. 229, Pruning Lowbush Blueberry Field, University of Maine Cooperative Extension.
  • Wild Blueberry Fact Sheet No. 219, Disease Management Guide For Lowbush Blueberries, University of Maine Cooperative Extension.
  • Wild Blueberry Fact Sheet No. 211, Blueberry Diseases 1, University of Maine Cooperative Extension.

 Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 1990

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