By Swingle Plant Pathologist Steve Geist:
Here are a few insects and diseases making news in Colorado landscapes…
Emerald ash borer:
Likely, you have heard about emerald ash borer being detected in Boulder, Colorado. This surprising news first broke during the last days of September in 2013. Surprising in that the next closest known infestation was Kansas City over 600 miles away. It is the most destructive insect pest to ever come in contact with North American urban forests. Initially, five trees were found, then a group was discovered at the University of Colorado in Boulder, and most recently another outbreak just south of the university campus. So now the questions are:
Just where is the emerald ash borer?
How long has it been in Colorado?
When will large scale infestations kill many trees?
Within the city limits of Boulder, an exhaustive survey is underway to determine where the insect is. Within the next few months, the results of the delimiting study will give us an answer for the City of Boulder. In other communities, tree care professionals are diligently looking for emerald ash borer. To date, it has not been found outside of Boulder.
Questions two and three are related or we believe they are related. In the Midwestern United States where emerald ash borer has caused massive destruction, epidemiology curves have been developed. There is a phase where the insect begins to multiply in the tree, there is a phase where massive numbers of untreated trees die, and finally there is a phase where the borer may have eaten itself out of house and home. So the question is – where are we in the cycle? To determine this, scientists who study tree growth rings are examining emerald ash borer infested wood from Boulder. These scientists, dendrochronologists, are looking for stark reductions in the tree growth rings as a sign of when borers first challenged the tree. Not surprisingly, the scientists are having trouble with the Colorado trees. As we know, there are many environmental factors that beset our trees including drought, defoliating hail storms, freezes, and other insects that also cause decrease in tree growth. Currently, the dencrochronologists believe that emerald ash borer may have been in Boulder for up to six years, however research continues.
If, and that is a BIG if, emerald ash borer has been in Boulder for six years and if models from the Midwest hold true, then we have a few more years before mass numbers of untreated ash succumb to borer infestations. The corollary is, if the insect has been here for six years then likely it is in some other communities – it just has yet to be discovered. The prediction for 2014 is that emerald ash borer will be found in additional locations. The good news, if there is any, is that effective control measures are available to protect your trees from emerald ash borer.
Downy Mildew on Impatiens:
Downy mildew isn’t a household word, but is also a new 2013 Colorado entry that will impact many 2014 annual flower displays. Downy mildew is a fungus that resides in the soil. The disease infects the entire plant turning impatiens into mush. Control for this disease begins before spring planting with a soil treatment. Sprays will also need to be completed during the summer. Once the disease takes hold, there is no salvaging the bed of impatiens. The plants will need to be discarded, the soil treated or replaced and new plants installed. No question, Downy Mildew will be here again in 2014.
Spottedwing Drosophila:
Found in Colorado in 2013, the spread of this insect will cause major problems for home raspberry and strawberry growers. Spottedwing is a close relative to the common fruit fly. You know, those annoying flies found buzzing around over ripe fruit. But spottedwing drosophila attacks fresh fruit and most commonly berries such as raspberry and strawberry. The fly lays eggs in the fruit with the developing insect essentially destroying the crop. Keep an eye out for this one. Keep your berry plants clean of debris and overripe fruit. There are insecticides available, but options are limited as this is a food crop.
Pine wilt nematode:
This one has been in the state for a few years, but really didn’t cause much of a stir until 2012. A wood boring insect spreads a tiny worm-like animal called a nematode to healthy pine trees. The nematode multiplies and clogs up water conducting tissues. In the late summer and fall, infested trees die rapidly. This is a very devastating disease of Scotch and Austrian pines readily found in central and eastern Kansas and Nebraska. In 2012’s unusually hot summer, we saw a definite uptick in tree mortality. We sent many samples to Colorado State University’s plant diagnostic lab and they positively identified nematodes as the causation for demise. The good news is that in 2013, we saw only a few trees that died from pine wilt. If our 2014 spring and early summer are especially warm the incidence of pine wilt will increase. If cool, maybe we will likely get a respite. Preventive controls are available, however limited to a trunk injection with a nematicide.
Thousand Cankers of Walnut:
A tree disease that is effecting Denver and surrounding areas is thousand cankers. Colorado black walnuts started to die in 2003, but it wasn’t until 2008 that researchers found the fungal cause. A tiny beetle less than 1/16th of an inch spread canker fungus underneath the tree bark. Thousands of cankers develop, grow together and kill the tree. Colorado Springs and Boulder were the first communities impacted by this disease. Western parts of the Denver metropolitan area have been working with thousand cankers for several years and is now becoming prominent on the east side as well. Most recently, walnuts in Fort Collins, Greeley, LaPorte, Berthoud, and Loveland have become infested. What we have seen in these communities is that initial mortality of trees is high while select walnuts appear to be resistant. In communities where the disease has been an issue for several years, should see fewer incidences. Where the disease has just cropped up thousand cankers will continue to cause walnut mortality. Chemical controls for thousand cankers are thought to slow progression, but do not stop the disease.
