Year in Review

Features - Plant Health

From wicked weather patterns to boring beetles, 2012 was an interesting year.

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December 7, 2012

The season of 2012 was many things to many professionals in the green industry throughout the country. The economy was difficult but improved for many individual green industry companies. Invasive species reared their ugly head in many ways, from the spread of emerald ash borer in new states to the major increase in impatiens downy mildew — check out www.endowment.org — in many areas of the country. Environmental stress played a key role in plant health in a myriad of ways, including effects of drought for many. With all of this, remember we are still the industry that taps into and nurtures the ultimate greening of America, the ultimate connection to solar power: photosynthesis.


Déjà vu all over again

For our summary, let’s first take a look at an example from 2012 that illustrates the connection of the present to the past. Environmental conditions and their role in plant health are commonly overlooked but vitally important. This lesson was brought home early in the decade of the 2000s when we had the second wettest year recorded in Ohio followed by the 3rd driest year the following growing season. This one-two punch resulted in severe damage to many woody plants, including Taxus (yew). Large Taxus specimens, many of which had thrived for 40 years or more at the Secrest Arboretum of OSU’s Ohio Agricultural Research and Development Center in Wooster showed thinning, foliar discoloration, browning, and in some cases, death in the second year — the summer of drought.

This seemed curious, since we associate Taxus with problems with wet conditions, often saying that Taxus “cannot tolerate wet feet.” We excavated some of the large Taxus at Secrest (some were specimens of 20-by-20 and larger) and noted considerable root and crown rot from the water mold, Phytophthora cinnamomi. This same scenario also played itself out at a number of other arboreta on the East Coast and Midwest with similar large, well-established Taxus specimens. What was up? Why were these Taxus plants dying in dry, rather than wet years?

Bark beetles invade elm bark.

What we suspect happened is this: that P. cinnamomi was present in these soils all along, fighting a rhizosphere war with the extensive root systems of these large Taxus plants. Over the years, the root systems prevailed, losing some roots, but having enough healthy roots to survive and thrive and supply the plants with adequate water and minerals. The very wet year resulted in severe stress to the roots and thus the plants, but they survived. Then the next season, when a good growing season might have resulted in recovery and production of adequate new roots to turn the tide, the early drought prevented adequate recovery, and we lost many of these Taxus, even though they had weathered the vicissitudes of nature for decades. The back to back punches of near —record wet and then dry seasons tipped the scales.

Can we prove this “interesting, if true” story? No, but it does fit our knowledge of how soil pathogens such as Phytophthora spp. behave in terms of it not being an either-or situation relative to overall root system health. There is ever an organismal struggle between roots and their pathogens with the upper hand depending upon many factors including the environmental conditions and how these affect both the roots and the pathogens.

Tunneling from bark beetles.

And one important note, last year northeast Ohio and many other Midwest areas suffered among their wettest years ever, then — déjà vu all over again — this year we are having the same pattern of early droughty conditions, occurring until late July. The result: virtually all of the remaining large Taxus specimens in the Taxus plot that survived the earlier one-two punch became discolored, thinned and either died or were rendered horticulturally untenable and were pruned at ground level. A tale of Taxus travails over time, applicable to many plants and the role of both short- and long-termed environmental stress on plant health.


Beetlemania

Let’s now turn our attention to several insects that often correlate to environmental conditions and often confuse us in terms of identification.

Classic frass “toothpicks” from ambrosia beetle.

Bark beetles and ambrosia beetles may belong to the same family (Scolytidae); however, these two distinct groups of borers differ in their hole-making behavior and their larval feeding activity. Knowing the differences are important to identifying the true culprit, which is helpful to establish when an infestation was initiated — did the borers infest trees in the nursery or in the landscape after they were planted? Here are a few things that these borers have in common. Both types of borers are tiny 1/8-inch long beetles, and both produce shot-sized holes in the bark. While some species of bark beetles and ambrosia beetles will only infest dying or dead trees, other species of both types of borers will target stressed trees; the 2012 drought generated a considerable amount of borer fodder.

Here are the differences. Bark beetles feed on phloem tissue. Females bore through the bark and tunnel into the phloem where they feed and lay eggs. The resulting larvae confine their feeding activity to the phloem. Once the larvae complete their development, the new crop of adults emerge directly through the bark producing a new, and usually more numerous, set of shot-holes. So, if you strip away the bark and only find tunneling in the phloem, it’s bark beetles. This means the infestation was initiated some time ago.

Ambrosia beetle females tunnel through the bark and bore into the xylem. As they tunnel, they push a sticky mixture of excrement (frass) and wood particles backwards out their entrance holes; the extruded mixture are commonly described as looking like “frass toothpicks.” The females release “ambrosia” fungi that colonize the xylem and their larvae do not eat wood; they eat the fungus that grows from the walls of the tunnels created by the adults. Once the larvae complete their development, which occurs deep within the xylem, the new crop of beetles make their way out of trees using the same tunnels and holes created by their parents; they do not produce more shot-holes through the bark. So, if you strip away the bark and find holes bored into the xylem, it’s ambrosia beetles. If you see frass toothpicks, the infestation is just starting.

 

Ambrosia beetle tunnels.

Solar power
Enough of parasites, pests and problems. Remember that plants, are the truly “green” industry. Let us not cede the public forum on greening to those speaking of solar panels. Let us tout the beauty of the true meaning of energy on this planet, the beginning of the food chain, the capturing of solar energy by chlorophyll in plants, combining this energy with carbon dioxide and water in photosynthesis which drives energy production in plants and animals that eat plants. The green industry indeed. As Robert DeFeo of the National Park Service said, “The most important chemical reaction on earth is photosynthesis...We are all parasites upon it.”


 

Jim Chatfield is assistant professor and extension specialist, Department of Horticulture & Crop Science/Department of Plant Pathology; and Joe Boggs is assistant professor and extension educator, Hamilton County and Department of Entomology, Ohio State University; chatfield.1@cfaes.osu.edu, boggs.47@osu.edu.