{"id":1225,"date":"2010-06-08T11:11:24","date_gmt":"2010-06-08T15:11:24","guid":{"rendered":"https:\/\/extension.umaine.edu\/blueberries\/?page_id=1225"},"modified":"2025-09-09T13:07:55","modified_gmt":"2025-09-09T17:07:55","slug":"pruning-methods-on-disease-and-insect-control","status":"publish","type":"page","link":"https:\/\/extension.umaine.edu\/blueberries\/resources\/disease\/pruning-methods-on-disease-and-insect-control\/","title":{"rendered":"218-The Influence of Pruning Methods on Disease and Insect Control"},"content":{"rendered":"

Fact Sheet No. 218, UMaine Extension No. 2238<\/strong><\/p>\n

\n

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.\u00a0 January 1990.<\/em><\/p>\n

Introduction<\/strong><\/h3>\n

There are several effective methods to prune lowbush blueberries:\u00a0 burning with oil or propane-fired burners, straw burning, free burning (generally less effective), and flail or rotary type mowers.\u00a0 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.\u00a0 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.<\/p>\n

In many instances, pruning can be accomplished with flail type mowers, rather than more costly traditional methods of oil or straw burning.\u00a0 When fields are mowed, they need to be continually and closely examined for disease and insect infestations.\u00a0 Expanded monitoring by trained scouts has led to increased awareness of potential problems.<\/p>\n

Pruning with mowers is a fairly new management practice.\u00a0 In the early 1980s, research at the University of Maine demonstrated that flail-mowing, if done properly, is as effective as burning.\u00a0 Pruning with mowers costs less, uses less non-renewable fuel (than oil burning) and eventually increases the organic matter on the soil surface.<\/p>\n

However, pruning with fire offers advantages that mowing does not.\u00a0 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.\u00a0 Fire pruning may also destroy overwintering fungal organisms, such as the Mummy Berry fungus, and certain fruit rotting organisms, which live on dead leaves and stems on the soil surface.\u00a0 Traditionally, lowbush blueberry growers have used fire to prune. As a result, diseases and most pest insects have not been a major problem.<\/p>\n

Diseases<\/strong><\/h3>\n

Mummy Berry (Monilinia blight<\/em>) disease has been the most destructive lowbush blueberry disease in Maine in the past five years.\u00a0 It has been an economic threat to many fields that have been flail-mowed for multiple cycles.<\/p>\n

Weather and field conditions have also affected the incidence of diseases.\u00a0 Wet weather during bud break or high water tables can contribute to outbreaks of Mummy Berry spore production and infection in burned or flail-mowed fields.\u00a0 Dry weather, on the other hand, promotes spore release from infected tissues.\u00a0 Botrytis blight<\/em> is most severe during mid-to late-bloom, when cool, rainy weather follows a frost.\u00a0 Other diseases, such as red-leaf and powdery mildew, have been prevalent but have not been linked to crop losses or mowing practices.<\/p>\n

Recent studies at the University of Maine Agricultural Experiment Station show differences in disease incidence because of pruning practices.\u00a0 Five fields with side-by-side comparisons of pruning treatments were closely monitored for Mummy Berry, powdery mildew, and red-leaf diseases.\u00a0 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.\u00a0 Mummy Berry disease 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.\u00a0 These results indicate that Mummy Berry disease does increase over time with flail-mowing and that a short return to burning will reduce the disease but will not eradicate it.<\/p>\n

In 1989, the fruit was harvested from the same test fields.\u00a0 The incidence of fruit rot diseases was greatest on berries harvested in the flail mowed fields.\u00a0 The incidence of Botrytis <\/em>was not affected by pruning method. (Botrytis <\/em> spores tend to be everywhere and have not been reported to build up due to pruning method.).\u00a0 The fruit rots that are at higher levels in flail-mowed fields are Glomerella<\/em> (anthranose) and Altenaria. <\/em>Both of these diseases affect fresh and perhaps frozen fruit quality.<\/p>\n

Growers who choose flail-mowing over burning should monitor their fields for the incidence of Mummy Berry disease and take corrective measures to avoid economic losses.<\/p>\n

Insects<\/strong><\/h3>\n

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.\u00a0 Economic damage by spanworm larvae and, to a lesser degree, by the flea beetle has been observed on flail-mowed fields.\u00a0 These insects have also been found in “lightly” burned or “scorched” fields.<\/p>\n

Spanworm larvae have been found in consistently greater numbers in flail-mowed fields than in burned fields.\u00a0 Commercial growers have experienced extensive outbreaks since 1987 on fields that have been flail-mowed for multiple cycles.<\/p>\n

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.<\/p>\n

The spanworm and flea beetle can seriously threaten blueberry production if populations become high.\u00a0 Both these insects feed on leaves and fruit buds and are difficult to control due to their presence at pollination.\u00a0 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.<\/p>\n

Sawfly has also appeared in consistently higher numbers on flail-mowed fields.\u00a0 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.<\/p>\n

Monitoring for Pests<\/strong><\/h3>\n

Whenever a management practice is changed, it will usually change some other factor in the production system.\u00a0Certainly, flail-mowing has had a marked effect on the incidence of disease and insect infestations.\u00a0 Producers should monitor their fields with increased intensity to detect outbreaks before they affect profits.<\/p>\n

Action thresholds are very helpful when monitoring pests.\u00a0 (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).\u00a0 It is very important to know at what point it may be necessary to control a pest.\u00a0 Trying to kill every pest in a field is expensive, impossible and may lead to resistant pest populations.\u00a0 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.\u00a0 Thresholds have not been developed for every blueberry pest, but those that are available can be helpful guidelines.<\/p>\n