by William Nesmith
The widespread planting of Tennessee 90 this season played a key role in minimizing losses from outbreaks of the aphid-borne virus complex in burley tobacco that occurred this fall. Losses statewide due to the virus complex have been significantly below what was expected, considering that so much of the 1997 crop was planted late and losses in some varieties highly susceptible to the virus complex have been very high. Recall that in mid-August I had predicted losses could be high because the virus complex was increasing rapidly and much of the crop was late. The actual loss due to virus is much below what I had expected.
What happened? Virus resistant varieties made up a much greater percentage of the crop planted late than I had anticipated. Our information indicated that much of the late crop was being set with hold-over plants of what ever variety growers could find. Therefore, I had assumed the variety-mix would be similar to that of 1996. What I did not know was the magnitude of variety shift that had occurred between 1996 and 1997, especially those intended for mid-season plantings.
Tn 90 was the number one burley cultivar planted, especially targeted for mid and late-season crops. Tn 90 is resistant to the aphid-borne virus complex as are many other varieties, but it also has some blue mold tolerance. It is interesting to note, however, that the majority of growers interviewed about their variety decisions, cited blue mold tolerance as the main reason for changing to Tn 90, rather than its virus resistance. Apparently, many growers were selecting any advantage they could find against blue mold after their experiences of 1995 and 1996, especially for mid and late-season crops that had experienced greater losses from blue mold. Most were aware they would not be prepared to spray foliar fungicides and needed any advantage a variety would grant.
It is even more interesting to note how the decision has changed on some farms, where multiple cropping and double- barning is practiced, and the growers were reasonably knowledgeable about the virus risk. Several of those growers stated that they had used 14 x L8 for their late crop until 1997, primarily because of its early maturing potential and just accepted the virus risk associated, but they had elected Tn 90 (medium maturing) this year because of its tolerance to blue mold; accepting the higher risk of frost damage with a later maturing variety. Virus resistance was accepted as a bonus not as a primary factor in their decision. Probably of even greater impact on losses from viruses this season, were these same growers' decisions with mid-season varieties, where viruses usually play a minor role. Due to the large amount of crop set after June 20, the stage could have been set for much greater losses from viruses. For example, many cited changing from hybrid 403, highly susceptible to blue mold and the viruses but with high yield and quality, to Tn 90, which has lower yield and quality potential but is tolerant to blue mold and resistant to the viruses. In fact, these mid-season plantings ended up being planted significantly later than planned and had they had the same variety mix as in 1996, losses from the viruses could have been great. As a result of these developments, many had resistance to the viruses in place at the right time. Keep this in mind as future variety decisions are being made.
by Paul Vincelli
I've had questions recently about how to select alfalfa varieties with appropriate types and levels of disease resistance. There is a very simple rule of thumb: Select high-yielding varieties with an R or HR rating to Aphanomyces root rot. I'll explain the basis for this.
First, start by identifying the high-yielding varieties. Examine the alfalfa variety test report produced annually by the UK Dept. of Agronomy. That publication neatly summarizes variety tests over the entire state for several years. Unfortunately, there is no single variety that always is among the top yielders in every location, every time. Life never seems to be that simple. However, there ARE varieties that GENERALLY perform in the top group across many locations and years. Aren't those the ones you want?
But what about the other diseases we sometimes talk about: Phytophthora root rot, anthracnose, bacterial wilt, Fusarium wilt? Yes, we still recommend resistance to all of these. In particular, Phytophthora root rot is a very important disease on many farms where alfalfa is being produced. But the situation is really very simple. If you select a variety with an R or HR rating to Aphanomyces root rot, it will already have adequate levels of resistance to these other diseases. The Aphanomyces resistant varieties are the "new kids on the block", and they've all been developed to already have resistance to these other diseases. Convenient for farmers, isn't it?
by John Hartman
Recent weather reports confirm that we are experiencing dry weather throughout most of Kentucky. Although not as dramatic and noticeable as our major summertime droughts of recent years, this dry weather can have an effect on landscape plants. Homeowners and landscape managers may not be noticing dry weather effects on trees and shrubs because the leaves are turning color and falling this time of year anyway. At risk are evergreens, because they transpire year-round; recently transplanted trees and shrubs, because they lack extensive root systems; and mature trees growing on poor quality or shallow soils. Dry weather in autumn increases 1) susceptibility to many diseases and 2) sensitivity to winter injury.
Increased disease susceptibility. There are several twig and branch canker diseases of landscape trees and shrubs that normally do not appear until after a drought has occurred. Drought-related predisposition to attack by opportunistic pathogens can occur even when drought stress symptoms are not obvious. The role of water stress in encouraging opportunistic plant pathogens is unclear, however, the stress condition may interfere with the plant's defense against such pathogens.
Certain fungi such as Hypoxylon, cause of limb and branch cankers of oaks, and Armillaria, which attacks and decays the roots and base of many woody plants are thought to respond to drought stress. Similar relationships to drought may exist with other fungi including Thyronectria, cause of honey locust canker; Cytospora or Valsa, causes of cankers on spruce and other conifers, poplar, willow, maple, and ornamental cherry, plum, and peach; and Botryosphaeria, cause of cankers of redbud, rhododendron, and many other woody plants. Symptoms of these cankers may not appear until the season following the dry weather. Pine tip blight, caused by Sphaeropsis sapinea is more severe on drought stressed Austrian pines.
Foliar diseases such as powdery mildew are already causing scorch and leaf rolling symptoms on many dogwoods. Dogwoods with adequate soil moisture show fewer of these drought symptoms, and should enjoy better long-term health than unwatered dogwoods. Dogwoods are not the only plants sensitive to dry weather. Burning bush, sugar and Japanese maples, hemlock, spruces, white pine, birch, and taxus also decline after dry weather.
Winter injury sensitivity. Woody plants need soil moisture going into the winter. Lacking water, many plants show increased susceptibility to winter injury. The winter injury may take the form of tip burn of needle evergreen foliage or marginal scorching of broadleaved evergreens, sunscald and "frost cracking" of tree trunks, or of browning and death of the cambium and phloem tissues following cold weather.
What to do. Valuable plants need to be watered to prevent stress-induced canker diseases and to avoid winter injury. Be prepared to prune out dead and injured branches next year. More information on this subject is available in U.K. Extension Publication ID-89, "How Dry Seasons Affect Landscape Plants."
Lee Townsend
Extension Entomologist