This is the fourth part of a series on summer tree diseases. This article examines beech bark disease, and black rot of apple.
During the summer months, plants are in the midst of their development. While many plants flourish due to the warmer climate, others can be subject to infections from a slew of disease pathogens. The following describes some of the most common diseases to affect plants in summer, and how they impact their hosts.
Beech Bark Disease (Neonectria faginata and Neonectria ditissima)
Beech bark disease is an insect-disease complex responsible for causing significant mortality and defect in American beech (Fagus grandifolia) and European beech (Fagus sylvatica). Beech bark disease results when the beech scale insect, Cryptococcus fagisuga, infiltrates and alters a tree’s bark, creating a wound. The wound allows two different fungi, Neonectria faginata (previously called Nectria coccinea var. faginata) or Neonectria ditissima (previously called Nectria galligena) to invade the tree, causing a canker to form. Over subsequent years, cankers continue to form, eventually killing the tree.
Beech bark disease infects American beech and European beech. It can be found in landscape settings, but is especially prevalent in mixed-hardwood forests, as well as forests comprised primarily of beech trees.
Symptoms of Infection
Initially, the most conspicuous symptom is the white wax secreted by the beech scale insect. White wooly dots will appear on roughened areas of the bark, below large branches, and beneath mosses and lichens. As the insect population increases, the entire bole of the tree may be swathed in white. Dead spots on the tree are indicative of fungal infection. On some trees, a red-brown exudate may ooze from the dead spots. This exudate is often referred to as slime flux. Perithecia will often begin to form around the dead sections of bark. The inner layers of infected bark turn an orange color. Fungi may infect large areas on some trees. Severe infections can create a girdling effect, causing the bark to redden. This often results in significant foliar dieback. On declining trees, leaves that emerge in spring will be stunted, turning a pale yellow color.
Trees that have been infected by beech bark disease are susceptible to invasion from other insects, and wood-rotting pathogens. Ambrosia beetles are attracted to weakened beech trees, and create perforations that allow other fungi to enter. Species of Hypoxylon that decay sapwood, and the shoestring root rot fungus, Armillariella mellea, will sometimes invade affected trees, and hasten their decline.
- Biological control can be effective in isolated areas. A ladybird beetle, Chilocorus stigma, feeds on scale, and may be utilized to reduce insect populations. A fungus called Nematogonum ferrugineum, which parasitizes nectria fungi, can be employed to combat fungal infections.
- On high-value ornamental trees, beech scale insects can be controlled using horticultural oils. Two applications should be performed: one spraying oil application, and one larval spray application.
- Power washing trees infested with beech scale can help to minimize populations.
- Salvage cutting is the most ideal method for forested settings; this will help to prevent the spread of beech scale to other trees.
- At present, there are no treatments available for the fungal pathogens.
Black Rot of Apple (Bostryosphaeria obtusa)
Black rot of apple, also called frogeye leaf spot, is a fungal disease that infects the fruit, leaves, and bark of various hardwood trees, including apple, pear, and quince. The disease is caused by the fungal pathogen Bostryosphaeria obtusa. The fungus is pathogenic on fruit trees. On most other hardwoods, the fungus is a secondary invader, or a saprophyte, colonizing and thriving on dead or weakened tissue. Black rot infections result in the formation of leaf spots and cankers on branches, while also causing fruit degradation. On infected hosts, loss of fruit yield and extensive defoliation from leaf spot is common. Severe infections can reduce tree vigor, rendering hosts more susceptible to invasion from other disease pathogens and insects.
Black rot primarily infects apple, cranberry, pear, and quince. As a secondary invader, black rot may infect alder, apricot, amelopsis, birch, bittersweet, catalpa, chestnut, cotoneaster, currant, dogwood, elder, hawthorn, hophornbean, lilac, linden, maple, mountain ash, mulberry, oak, peach, persimmon, rose, sassafras, sumac, tree of heaven, and witch hazel.
Symptoms of Infection
Initial fruit infections occur during the bloom period. Symptoms of fruit infection will generally become apparent in mid-summer, as the fruit matures. Decaying tissue with brown and black concentric rings may be observed on large sections of infected fruit. Some fruit will become desiccated, and remain attached to the tree. Occasionally, fruit will ripen prematurely, with the rotten tissue concentrated at the core.
Leaf infections develop as the bloom period concludes. Infected leaves will develop circular lesions with purplish or reddish outer borders, and light tan interiors. Cankers appear as sunken, reddish brown areas on infected branches and twigs. Cankered sections of the tree will often have rough or cracked bark. The black rot fungus is frequently observed in plant tissue that has been killed by fire blight, or is undergoing environmental stress.
- Prune out and dispose of dead or diseased plant parts on susceptible trees. Administer pruning cuts at least fifteen inches beneath infected tissue. Infected plant parts can be burned, buried, or composted. Pruning should be performed during winter, when the fungus is inactive.
- Control fire blight by excising infected wood from plants.
- Maintain plant vigor by reducing stress from environmental conditions, and limiting insect infestations.
- Ensure that plants are sufficiently watered, especially during extended periods of drought.
- Apply a layer of organic mulch around the base of susceptible plants to improve soil quality, moderate soil temperature, and retain soil moisture.
- Remove the stumps of infected trees that are culled. Dead stumps may serve as overwintering sites for the disease pathogen.
- Fungicidal sprays may be utilized to prevent the infection of leaves and fruit. Initial applications should begin in early spring, during leaf expansion. Thoroughly drench the leaves and fruit to ensure efficacy. Susceptible branches may still be infected by the spores.
Photo courtesy of Public Image Library CC-by-2.0