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Nebraska's Forest Health Report

June 2019
Automated Watering Systems
The line where the sprinkler system has been hitting this tree is clearly visible
While automated sprinkler systems can be convenient, they need to be frequently inspected to ensure they are hitting the desired target. When the constant stream of water comes into contact with branches, it can lead to fungal development and branch death. 

Trees do not require the frequent shallow watering that these systems provide, and instead benefit from more spaced out deep watering. The recommended watering schedule for trees is usually 1 inch per week in clay soils, and 2 inches per week in sandy soils if it has not rained during that time. Slow deep watering is key for proper tree health, but be sure to check the soil moisture beforehand.  

 
Hail Damage
Large hail is not uncommon this time of year in the Great Plains. Most trees will show some evidence of past damage from this, however most will recover if damage is not severe. Extensive hail may strip away large portions of the bark, which may lead to dieback. These stressful events may also open up a pathway for latent infections of canker-causing fungi or other fungal pathogens.

Depending on the type of infection, treatments may not be helpful. For example, new pine shoots can be sprayed with fungicides to prevent new Diplodia blight infection, but latent infections of Diplodia already in the branch will not be controlled with fungicides. Likewise, any cankers will be largely untreatable. After a hail event it will be vital to keep the tree properly watered and mulched to aid in recovery. 
These ponderosa pines north of Rushville, NE were hit by large hail in 2018 and are now beginning to show signs of Diplodia. Diplodia blight often occurs in pine branches as a latent infection, causing damage only when branch tissue is injured such as from hail.
Bagworms Emerging
These native pests will soon be hatching out of the bags they overwintered in (if they have not already done so). This means that manual control (hand picking bags) will no longer be an effective means of control. Instead, foliar sprays of insecticides such as Bt or Spinosad can be used to control young larvae.

Young larvae are extremely small and may be difficult to see at first. They construct tiny bags covered with bits of foliage, so examine trees closely for tiny cone-shaped objects moving about on the foliage.

Newly hatched larvae also use a process called ballooning, where they catch the wind with a silken strand to move to new host plants. Light infestations are unlikely to cause much damage. However, high infestations can be devastating, especially to evergreen species. Once larvae seal themselves in bags later in the year, insecticides will no longer be effective.  

 
Newly hatched bagworms are small and can be hard to see. 
Fungal Leaf Rusts
Leaf rusts appearing on different tree species.
(Clockwise from upper left): hawthorn, ash, buckeye, and pear.
This spring has been an incredibly wet year for most parts of Nebraska. These conditions tend to favor fungal leaf pathogens such as rusts, which will begin to make their presence known on impacted species. Some of these such as cedar-apple rust and its closely related species can be particularly damaging to non-resistant varieties of apple. Other species such as ash leaf rust, are usually only an aesthetic issue. 

Spores transported by wind will land on new foliage or buds in the spring, and disease symptoms begin to become apparent as the leaves grow. In some cases this will only cause discolored spots on the leaf. However, on particularly susceptible species, it may lead to defoliation or early leaf drop. As mentioned, with some such as ash rust, little will need to be done. However, others may require a treatment with an anti-fungal compound before bud-break in the spring. Care should be given when selecting new trees to ensure a resistant variety if that species is known to have problems with rusts. Visit http://extensionpublications.unl.edu/assets/pdf/g1907.pdf for more information. 
Fire Blight
Fire blight is a bacterial infection that affects mainly members of the rose family. The disease can be particularly bad in years with warm wet springs. The bacteria create a distinct "shepherds crook," wilting leaves and fruit, and a bacterial ooze. The ooze attracts insects which can spread the infection to new plants, along with rains, and movement through the already infested stem. Apples and crabapples can be particularly hard hit by this disease.

Infested tissue should be pruned out at least 8 -12 inches below where the diseased tissue is located. Take care to sterilize tools between cuts with a solution of bleach or alcohol to prevent spreading the pathogen. Additionally, treatments containing the active ingredient Streptomycin can be sprayed on the buds during the pink stage (3-4 days before bloom) and every 5-6 days after while blossoms are still on the tree. However, good control is difficult. To avoid future problems consider consulting your extension educator for resistant varieties when planting. 
Left - Distinctive shepherds hook of the stem (Credit:  Ward Upham, Kansas State University, Bugwood.org)
Right - Fruit producing bacterial ooze (Credit: A.L. Jones, University of New Hampshire) 
Pear Blister Mite
Pear blister mites are eriophyid mites which can cause significant damage and disfigurement to susceptible pear leaves. Mites overwinter under bud scales and emerge the following spring to feed on new growing tissues. Control can be achieved with monitoring and application of dormant oils in the fall when mites are found in the bud scales. Pyrethroid insecticides should be avoided as this can make infestations worse by removing natural predators. Pears with russetted surfaces may also show some resistance to infestation. 
Pear leaves showing evidence of blister mites in Southwest Nebraska. Mites are found in the blisters on the bottom side of the leaves. (Credit: B. Hoffman)
Compiled by David Olson, Forest Health Specialist
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