Inspector Findings in Kentucky

Our Area Code is Changing

Just a reminder that beginning April 1 our area code will change to 859.  You will, however, still be able to use the 606 area code until October 1. 

How Will Landscape Plant Diseases Respond to the Drought of 1999?

by John Hartman, Extension Plant Pathologist

In contrast to the drought effects on diseases of annual crops discussed in the accompanying articles, drought effects on diseases of perennial plants can be very dramatic.  In the case of trees and shrubs or fruit crops, the drought has not only affected the pathogen but also the physiology of the host from one year to the next.  Host plant condition affects its reaction to disease.

Woody plants.  Most of us are familiar with wilting and leaf scorch symptoms associated with dry weather.  This past year, leaves of drought-stressed plants closed their stomata which reduced their rate of photosynthesis.  Reduction in photosynthesis may not kill a tree or shrub, but it means fewer carbohydrates are made and stored for future use.  In the landscape, seedlings and recently transplanted trees and shrubs were at greatest risk because they lacked extensive root systems.

With drought, there are some fungal diseases of landscape trees and shrubs that often do not show symptoms until the following season, after the drought has passed.  The role of water stress in encouraging opportunistic plant pathogens is unclear.  It is possible that the stress condition interferes with the plant's defense against such pathogens, or possibly, the reduced carbohydrate reserves allows the plant little energy to fight invasion by pathogens.

Expect certain fungi such as Hypoxylon, an oak pathogen, and Armillaria, which attacks many woody plants, to appear in 2000 because of the 1999 drought stress.  In addition expect symptoms of diseases caused by other fungi such as Thyronectria, cause of honey locust canker; Cytospora or Valsa, causes of cankers on prunus, poplar, willow, maple, spruce and other conifers; Sphaeropsis, cause of pine tip blight; and Botryosphaeria and Nectria cause of cankers of many woody plants such as rhododendrons, crabapples, dogwoods, maples, and others to appear the season following the dry weather.

In searching for water, some woody plants could have sacrificed surface roots to the drought while relying more heavily on roots that were deeper in the soil.  When the excessive rains return, partial flooding could render these deeper roots more prone to root rot diseases, thus leaving the woody plants with few functional roots.  Thus, expect additional woody plant death when the drought breaks.

One possible benefit of the drought could be the reduction in foliar diseases in the year 2000.  There should be less carry-over inoculum from shade tree anthracnose diseases, crabapple scab or dogwood powdery mildew, for example.  The benefit could be short-lived, however if spring weather is wet and rapidly repeating cycles of these diseases occur.  Looking ahead even farther, the rust infections of cedar that should have occurred, but didn't, during the dry 1999 summer might result in fewer cedar galls in the spring of 2001 and less rust on crabapples and hawthorns that same summer.

Herbaceous ornamentals.  Perennial flowers and ground covers, like their woody counterparts could have reduced energy reserves due to the drought.  This could make them more susceptible to cankers and to root, corm, or bulb rot diseases.  There is not much research on the role of stress on diseases of herbaceous ornamentals, so it is difficult to know how the drought will affect these plants.  A few diseases such as Volutella blight of Pachysandra, are known to be more severe on stressed plants, but most likely the disease would have appeared during the drought.  For foliar diseases, the situation is similar to that of woody plants - reduced primary inoculum might result in less disease, at first.

Use of Flowering Plants as Indicators for Timing Management Actions for Landscape Insect Pests

This article appeared in the May 1997 issue of Inspector Findings in Kentucky.  Since that time, there has been several new businesses come along so we thought that it would be good to give you this information again.

Guy Mussey and Dr. Dan Potter conducted a three-year research project where they monitored the emergence and activity of 32 important insect pests in Lexington.  The pest activity was then correlated with the flowering sequence of several dozen familiar landscape plants.  The result is a timetable that allows growers to anticipate appearance of key insect pests and more effectively time measures for their control. 

Note:  The approximate date was calculated by averaging when the pest was first observed in 1992-1994.  Try to avoid using only the calendar dates for timing treatments. Since insects are cold blooded, and like flowering plants, their phenology will be earlier or later depending on how cool or warm it is in a particular spring.  However, since both are driven by temperature, the dates of appearance of a particular insect pest should follow a predictable sequence that is correlated with flowering or leaf flush of particular landscape plants.    

* begin treatment at time indicated

** treatment for leafminers can be done either at adult flight or can be postponed for approximately 4 weeks and a systemic insecticide can be used to target the larvae

*** make one treatment for these borers. Treatment should be done approximately 10 days to 2 weeks following adult flight. Treatments should be made as a bark spray with an insecticide that has some residual action such as Dursban, Lindane, Thiodan.

**** make two treatments at three week intervals
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Bacterial Wetwood and Slime Flux is Different from Winter Pruning Sap Flow

by John Hartman, Extension Plant Pathologist

When some tree species such as maples and birches are pruned in late winter, sap will flow from the cut branches.  This is a normal wound response for these species and the sap may flow for several days or weeks.  Do not confuse the flow of sap with the flow of wetwood fluids that are associated with infections by bacteria.  Wetwood fluids may flow year-round and are often converted to slime flux.  Slime flux, then is the foul-smelling and unsightly seepage from wounds in the bark or wood of shade trees infected with wetwood. 

Symptoms and cause of wetwood.  Wetwood seepage originates from infections of the heartwood and inner sapwood by common soil-inhabiting bacteria such as Enterobacter cloacae (Erwinia nimmipressuralis).  There are several other bacterial species also associated with wetwood.  Wetwood bacteria are capable of growing anaerobically (without oxygen) in the internal wood tissues.  Methane and osmotic or metabolic liquids, two by-products of the bacterial activity, accumulate under pressure and are forced out of the tree through the nearest available opening, usually a trunk wound or branch stub.  Pruning a branch or taking a core with an increment borer can sometimes release the materials under pressure, squirting the worker with foul-smelling liquid and gas.

The wetwood fluid, as it flows to the wounded bark surface, is a clear, benign watery liquid containing several nutrients. On the surface it soon changes to a brown, slimy ooze, as a result of feeding by fungi, yeasts, bacteria, and insects.  This surface slime flux may kill exposed cambium and bark surface organisms as well as grass growing near the base of the tree.  It occurs most commonly on bacterial wetwood-susceptible trees, such as ash, birch, elm, horse chestnut, maple, oak, poplar, and willow.  Although slime flux development is seasonal, evidence of wetwood and slime flux-stained bark is visible anytime.  This slime is also different from the liquid associated with alcoholic flux and bleeding necrosis.  Alcoholic flux emanates from shallow wounds and persists only for a short time and bleeding necrosis, associated with infections of the trunk by Phytophthora is associated with infections of phloem and cambial tissue, not deep xylem tissues.

Wetwood-infected trees have an internal core of wood that is wet but not decayed.  These infected branch, trunk, and root tissues also have a high pH.  Wetwood-infected wood is resistant to decay by fungi.  Thus, wetwood-infected parts of the tree are biologically protected from decay by fungi.  The extent of wetwood spread in the tree may be limited by tree defenses; however, wetwood can spread into new tissues as new injuries occur.  Thus deep injection holes and pruning can expand wetwood infection.  Take care to avoid pruning live branches on infected trees.  There is no evidence that spread of wetwood bacteria on pruning tools leads to infections of new trees.

Disease management.  Thus far, no effective preventive or curative measure is known.  If the bark is being stained it is tempting to drain the slime flux away from the branch or trunk so that it drips on the ground.  Drilling a hole into the tree and inserting a copper or semirigid plastic tube would accomplish this; however, this results in additional wounding and the threat of expanded wetwood or decay should be considered.  Loose dead bark should be carefully cut away so that the area can dry.

Understanding Degree-Days

The following two articles and list of plant pests was taken from the University of Tennessee Agricultural Extension Service Publication 1623.  These articles discuss using degree-days to determine pest activity and when control measures should be applied.  The equation for calculating degree-days is simple and can be a very useful tool in insect monitoring.  To determine degree-days, add the daily maximum temperature to that day’s minimum temperature.  Divide this number by 2.  Then subtract 50 from this number (all of the insects in this list have a base temperature of 50°F).  Example:  Let’s say for January 1, the maximum temperature was 70°F and the minimum was 40°F.  We add those numbers together (70° + 40°=110) and divide by 2 which gives us 55.  55 minus the base of 50° gives us 5 degree days for that day.  If the number is negative, then a 0 is recorded. The procedure is repeated each day and the number of degree-days is tallied.  There is a chart included at the end of the list of insects that will help with record-keeping.  There is also a link to a degree-day calculator on our web page:

http://www.uky.edu/Agriculture/NurseryInspection/links.htm.  If you use this link, make sure to pick a station that is close to where you are located and set the base to 50.

Reason for Using Degree-Days

Degree-Days (DD) are a method of accounting for heat units. Power companies use cooling degree-days and heating degree-days to calculate how much energy a  customer needs to cool or heat a house. Plants and animals that do not regulate internal temperatures (often called (“cold-blooded”) vary in their physiological development, or metabolism according to what temperature they are subjected to. In short, these organisms develop rapidly at warm temperatures and slowly at cool temperatures.  Therefore, we can treat plants and animals like a house — the more energy (heat) added, the faster things happen.  Conversely, the cooler (less energy) the organism, the slower it develops. If this rate of development related to temperature can be determined, a prediction of insect and/or plant development or activity can be made.

Using DD as a predictor takes into account cool vs. warm weather. Calendar scheduling of controls will usually be too early or too late unless the year is an “average year.”

Temperature Thresholds

Fortunately, most plants and animals develop within a specific range of temperatures. If the organism drops below a certain temperature, called the lower threshold, no development occurs (see Figure 1). Above this lower threshold, the rate of development increases with temperature in an almost straight-line fashion.

Most organisms also have an upper threshold temperature at which development begins to deteriorate because of heat shock. If the organism’s temperature rises too far above this threshold, it will die. In nature, most insects and plants find habitats that have temperatures above the lower threshold for sufficient time to complete a generation of development, but rarely exceed the upper threshold temperature.

Several field crops and ornamental plants are occasionally grown outside their original habitats. Corn plants shut down their development above 86°F and Balsam fir tends to stop development above 90°F.  Unfortunately, most state crop-reporting services are based on corn DD models that have the relatively low upper threshold of 86°F.  Most insect pests and other trees and shrubs do not stop development until temperatures reach 100-110°F. 

In reviewing DD thresholds for many insects and plants, several lower thresholds seem to be common.  Most soil-dwelling insects and some cool-season plants (i.e. conifers, maples) seem to have lower thresholds of 40°F (5°C) or 45°F (7°C).  Most above ground feeding insects (turfgrass surface feeders and most tree/shrub scales and caterpillars) seem to have a lower threshold of 50°F (10°C).

For all practical purposes, associating insect activity and plant phenology with 50°F degree-days (DD₅₀) is generally satisfactory.

Methods of DD Calculation

There are many methods for calculating DD. The easiest method is to use the average temperature method (see Figure 2). This method calculates the day’s DD units by subtracting the lower threshold from the average daily temperature. Ave T minus lower threshold (50) equals DD50.

Degree-Day Targets for Ornamental Plant Pests

In 1988, Warren Johnson of Cornell University produced one of the most comprehensive lists of insects and mites that attack trees and shrubs and associated Degree-day (DD) activity periods. These DD periods were not developed using rigorous observations and model development. They were developed by taking yearly notes of insect and mite activities.

These notes were then compared to DD charts for those same years and a range of DDs (base 50°F) were recorded.

The original list of insects and mites has been reduced to those that are of importance to Tennessee operations (see Table 2). This table is part of a publication in an Adobe PDF file format, the table is on page 24.

The box below will be a regular feature in the newsletter.  This box will contain several cities across the state.  Use the city closest to you and use those degree-day totals to determine pest activity for your area.  As you can see, there can be quite a variation across the state.