Inspector Findings in Kentucky

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Landscape Plant Diseases Being Seen Now

by John Hartman, Extension Plant Pathologist

This is becoming an interesting year for diseases of landscape plants.  The effects of the April 9 Sunday morning freeze are still being noticed.  In addition, the wet weather of recent weeks has favored numerous fungal diseases. 

Freeze damage.  Although this abiotic disease damaged many hosts, the most dramatic symptoms are being seen on certain Gingko trees in exposed locations.  Trees more protected or favorably located were unaffected and have fully expanded leaves at this time.  Affected trees have leaves less than one-half inch long, and from a distance the trees truly appear to be in trouble.  A closer look at the leaves reveals browning along the leaf margins.  Trees with adequate starch reserves (already depleted by last year's drought) should be able to re-leaf and survive.

Homeowners may notice a lack of fruit developing on their apple trees.  They are not alone because many commercial orchards lost nearly all their fruit production on certain varieties.  If backyard apples are being sprayed, should one continue with fungicides when there is to be no fruit crop?  Yes, continue sprays for the next month if the tree is susceptible to scab, because to allow the tree to defoliate will reduce fruit production next year.

Fungi are causing foliar diseases of landscape trees.  Wet spring weather has provided at least two lengthy leaf wetness periods for severe apple scab (Venturia) infections, and several for moderate to light infections.  In addition to apples, susceptible crabapples are also showing apple scab symptoms.  Look for olive-green to brown leaf spots, leaf yellowing, and leaf drop.  In many cases, scab is severe, and leaves show dead blotches, sometimes referred to as "sheet scab."  Wet weather has also favored maple and sycamore anthracnose diseases (Kabatiella, Apiognomonia).  On maple leaves, look for individual angular brown spots, which run together, especially along the veins.  Sycamores show brown leaf blotches, and sometimes twig dieback.  In landscapes throughout Kentucky, many ash trees are littering the lawns with infected leaflets, each one showing one or more blotches where ash anthracnose (Discula) has been active.

On the lawn:  While picking up all those ash leaflets, take a close look at the grass.  In the tall fescue lawn, one should be able to notice rust and smut in individual grass clumps.  Rust (Puccinia) appears with typical orange pustules scattered on the leaf surface.  Stripe smut (Ustilago) causes long, narrow, black lesions running along the leaf blade.  These lesions are filled with black, powdery spores of the fungus.  Kentucky bluegrass, meanwhile, shows purple-brown oval spots on leaf blades, symptoms of Helminthosporium leaf spot (Drechslera). 

From the greenhouse:  When purchasing plants, avoid buying impatiens with circular yellow or brown leaf spots, dead foliage, and overall stunting.  They could be infected with a virus, impatiens necrotic spot virus (INSV), which is systemic and from which plants will not recover at home.  Other plants such as petunia may show circular brown spots on the leaves, and tomato transplants may show dead blotches on the leaflets, all caused by this virus.  Diseased plants should be excluded from the garden.

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Calico Scale Alert

By Mike and Dan Potter, University of Kentucky

Calico scales are being found in unusually high numbers on several landscape plants, including honeylocust, hawthorn, hackberry, dogwood and flowering crabapple. Infestations are so heavy, in some cases, that entire twigs and stems are covered by the scales and the trees are in decline.

Mature calico scales, Eulecanium cerasorum, are large, black and white globular-looking insects about the size of a pencil eraser. They have a soft, leathery body and when crushed ooze a gummy, wax-like fluid. The adult female is the life stage which is now being observed attached primarily to twigs and stems. Some people are mistaking them for ladybird beetles which are roughly the same size. 

Like other scale insects, the calico scale feeds by sucking plant juices. Heavy infestations can cause premature leaf drop, branch dieback and, coupled with other stresses, eventual tree death. It's hard to say why infestations are so heavy this year; scale insect populations tend to be higher when trees are already in a stressed condition (as they would be from last year's drought), and possibly due to mild winter and spring weather.      

The mature females will soon be dying. Underneath each of them, however, are literally thousands of eggs which soon will be hatching into crawlers. The crawler stage prefers to suck plant juices from the leaves (further stressing the plant) and also excretes copious amounts of honeydew. The sticky honeydew and resultant sooty mold are cosmetically unappealing and can stain patios and car finishes, and attract wasps and other nuisance pests. Another reason to take action against the crawlers is that they can become wind borne, spreading the infestation to other trees nearby.        

So What Should I Do?

It is too late to impact the mature females, which will be dying off soon anyway. However, the underlying eggs are due to hatch any day, probably this week or next. Observant tree managers can monitor the status of egg hatch and crawler emergence by flicking off the adult scales and observing the eggs, which look like a mass of fine flour. The yellowish, newly-hatched crawlers are tiny but their movement will be visible to the naked eye.

Insecticide applications, timed to coincide with emergence of young crawlers, will break the cycle of development and help alleviate further plant stress.  Conventional insecticides labeled for crawler control include Dursban, Malathion, Orthene, and especially synthetic pyrethroids such as Talstar, Scimitar, and Tempo(= Bayer Advanced Lawn & Garden Multi Insect Killer). Decent control of crawlers can also be achieved with 2% horticultural oil or insecticidal soaps. Thorough coverage of infested twigs, branches and adjoining leaves is important, and since the hatching period often lasts about a month, a second application probably should be made 2 to 3 weeks after the first.

Calico scales overwinter on the bark as mid-sized nymphs. To further reduce the likelihood of problems occurring next year, it probably would be wise to follow up with a dormant oil application (in fall or winter) to trees that were heavily infested with calico scales this year.

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Overnight Defoliation Of Oaks

by Lee Townsend, Extension Entomologist

Oaks can be stripped of their leaves practically overnight by May beetles.  Active now, these beetles are approximately 1" long and cylindrical, color varies from brown to black.

The species which attack oak feed at night, stripping the foliage and leaving only veins.  Consequently, the damage is present but there is no sign of the cause.  The beetles leave the trees during the day and may be found under leaves or grass around the tree.  Sevin is very effective against these insects.  The feeding period lasts for several days but one treatment should be sufficient. 

The larval stages are white grubs that feed on the roots of grasses.  Large expanses of turf or pasture can produce thousands of these beetles.  Fortunately, the beetles are around for only a short time and oaks will push out a new set of leaves.

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Yucca Plant Bugs

These brightly colored insects (bluish body red legs, long antennae, sucking mouthparts) can occur wherever yucca is grown.  Yucca plant bugs feed on sap and produce small light spots on the leaves where they have used their sucking mouthparts to feed.  Large numbers of spots can produce yellow areas.  Black spots of waste material are left behind.  The adults have wings but do not fly readily; they are more likely to run when the plant is disturbed.

A direct spray of Insecticidal Soap should provide control.  Repeat as needed.  Other insecticides such as Sevin (carbaryl) or Dursban (chlorpyrifos) may be used.  Check the label to determine whether or not the product can be used on yucca.

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Sod Webworms

North Carolina Cooperative Extension Service
North Carolina State University

Many species of sod webworms occur in the United States. The actual species present in any given area, however, is variable. Sod webworms feed on lawns, golf course grasses, some clovers, corn, tobacco, bluegrass, timothy, as well as pasture and field grasses. They usually favor bluegrass and 'Tifdwarf' hybrid bermudagrass but will attack most grasses.

Larvae cut off grass blades just above the thatch line, pull them into their tunnels and eat them. The injury appears as small brown patches of closely cropped grass. Where many larvae are present, patches merge into large, irregular brown patches.

Webworms overwinter as young larvae in silk-lined tubes a few centimeters below the soil line among the roots of weeds and grasses. During early spring, larvae feed on the upper root systems, stems and blades of grass. They build protective silken webs, usually on steep slopes and in sunny areas, where they feed and develop. In early May, they pupate in underground cocoons made of silk, bits of plants and soil. About 2 weeks later, adults emerge. Beginning in May, moth flights may occur until October. The moths, erratic and weak fliers, live only a few days and feed solely on dew. Active at dusk, moths rest near the ground in the grass by day. Presence of moths indicates a possible infestation of larvae. The eggs are laid indiscriminately over the grass and hatch in 7 to 10 days. Young larvae immediately begin to feed (mostly at night) and construct silken tunnels. The most severe damage occurs in July and August.

Most sod webworms complete 2 or 3 generations each year. About 6 weeks elapse between egg deposition and adult emergence. Infestations in lawns can be detected by applying 1 tablespoon of pyrethrin insecticide or 1 ounce of soap in 1 gallon of water per square yard. Sod webworms will surface among the blades within a few minutes at the border between living and dead areas of turf. If 3 or 4 webworms are found in a square foot, control is recommended. Because birds and other predators are not reliable in giving acceptable control, it is sometimes necessary to use chemicals to protect fine turf from further damage. Apply one of the following to the lawn but do not water it in. Use 6 gallons of spray mix per 1000 square feet except where noted. It is best to treat in the late afternoon as sod webworms feed in the evening.

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Is The Devastating Plum Pox Virus A Threat To Kentucky?

by John Hartman, Extension Plant Pathologist

Many of you are no doubt aware of recent reports of the discovery of Plum Pox Virus (PPV) in Adams County, Pennsylvania.  This is a destructive disease of stone fruit and could be a threat to peach, plum, and cherry production in Kentucky.  This virus has never been found in North America before, and now that it is here, we need to be aware of the findings surrounding this disease.  In this article, basic information about PPV will be discussed.  For more details visit the edifying web site on PPV, also called sharka disease, that is maintained by Pennsylvania State University.  This article is based on information obtained from the web site.

What was found?  Plum pox virus (PPV) was positively identified in a relatively small region of Pennsylvania in October 1999.

Where has the disease occurred before?  PPV was first found in Bulgaria in 1915, spreading gradually through Europe, reaching France in 1970, soon after, to England, and by 1984, to Spain.  Throughout Europe, plum pox is considered the most devastating disease of stone fruits, and it has been estimated that over 100 million European trees are infected. Plum pox continues to spread eastward in Eurasia and southward along the Mediterranean coast of Africa.  In the 1990's PPV was brought to Chile and within a few years, large numbers of trees in the stone fruit-growing regions were infected.  In North America, the only identified occurrence of plum pox is localized in 18 stone fruit blocks of 4 orchards in two townships in Adams County, Pennsylvania.  Due to the localized nature of this infection, it is hoped that eradication may be successful in eliminating this isolated focus of infection from North America.

Host range.  PPV infects not only plums but also all economically important stone fruit (Prunus) species including peach, nectarine, apricot, almond, and cherry.  PPV is also known to have the ability to infect some wild Prunus species, and a large number of weed species under laboratory conditions.  In Europe, it is believed that spread within orchards occurs from infected to healthy fruit trees. The role of alternate weed hosts, if any, in disease spread is not known, but needs further study.

Symptoms on Stone Fruit.  Symptoms of PPV may vary considerably with the plant species, the cultivar, tree age, nutrient status, and environmental conditions.  In addition, different strains or variants of PPV may vary in virulence, and thus disease severity.  Some infected plants show no clear symptoms at all.  Diagnostic symptoms on leaves may consist of light green discoloration bordering the leaf veins (vein banding) or chlorotic light green or yellowed rings on the leaf blades.  These symptoms may be obvious or barely visible to the eye, depending on factors described above.  Symptoms frequently are restricted to only a few leaves per shoot. Infected trees are not stunted and are difficult to identify. 

Fruits of peach and apricot may develop lightly pigmented chlorotic rings or line patterns resulting from several rings coalescing together.  Fruits may become deformed or irregular in shape, developing necrotic areas. The internal stone from an infected apricot fruit may show white to yellow or red colored rings on its surface when the flesh is removed. Plums are generally more severely affected and show more severe symptoms. For some plum cultivars, infected fruits drop prematurely from the tree. Infected plum fruits often develop darker rings or spots on the skin, are severely deformed, and develop a reddish discoloration of the flesh.  Affected fruit can be low in sugars and tasteless.

PPV infection of fruit trees results not only in development of typical symptoms on leaves and fruits, but also eventually debilitates the tree, reducing its useful life.  Unfortunately, many trees fail to show symptoms for the first few years following the initial infection of the tree. In the survey done this fall in Pennsylvania orchards, only 2 of the 18 infected peach blocks had trees showing obvious symptoms. Therefore, symptoms are not a good indicator of infection and cannot be relied upon to determine the incidence or range of the disease. When symptoms do occur, however, they are frequently very diagnostic and easily recognized.

The Plum Pox Virus.  PPV is a virus, an infectious agent much smaller than the disease-causing fungi or bacteria we often encounter.  To visualize PPV, one needs to use an electron microscope which can magnify objects to 100,000 times natural size.  Plum pox virus is a virus species in the genus Potyvirus.  Potyviruses are one of the largest families of plant viruses and representatives are found in Kentucky as potato virus Y, bean common mosaic virus, maize dwarf mosaic virus, tobacco etch virus, and watermelon mosaic virus, for example.  These viruses are transmitted by aphids.  Once PPV is inoculated to a plant by a vector aphid, the replicating virus can spread throughout the plant infecting all tissues, including leaves, fruits, flower parts, buds, young bark, and roots.  Plum pox virus is known to occur in several different forms or variants called strains.  The strain introduced into North and South America has been the PPV-D strain which appears to be more slowly spread by aphids in Europe, compared to other strains.

How does PPV spread in the orchard?  In orchards, PPV is spread only by aphids. Aphids are small insects that feed through modified piercing-sucking mouthparts on internal phloem tissues of plants.  One of the most efficient vectors, the green peach aphid (Myzus persicae) colonizes other stone fruits in Kentucky.  Therefore, the potential exists for aphid spread of PPV here.  The aphids transmit PPV in a non-persistent manner which means that once the aphid probes into an infected plant and acquires the virus, the virus can only remain infectious and be transmitted by the aphid for a short time (usually some minutes or an hour).

How can the virus move internationally, and how did it get here?  Long-distance spread of PPV by aphids is highly unlikely.  Usually, long-distance movement of tree fruit virus diseases is done inadvertently through commercial shipping of nursery stock or budding material, but that has been pretty well ruled out in this case.  A hobbyist propagator, a person who travels the world looking for new varieties from other countries, could bring a disease like this into the U.S.  How PPV was introduced into the U.S. is not known.

Control through exclusion.  Once PPV becomes established in a geographical region, it is very difficult or impossible to completely eradicate.  Therefore, it is important to prevent the introduction of PPV into the country.  Under the Plant Quarantine Act of 1912, in the United States, this is the responsibility of the Animal and Plant Health Inspection Service (APHIS) of the USDA.  All fruit nursery stock for importation is tested for a range of known fruit tree pathogens and especially for those that are not known to occur in the United States (exotic pathogens).  Only pathogen-free material is released for commercial use.  The occurrence of PPV in Pennsylvania serves to remind everyone of the importance and need of strict plant quarantine and testing procedures associated with imported nursery materials. In almost all cases, transoceanic dispersal of plant pathogenic agents is associated with human transfer of infected host materials.

Therefore, careful regulation and inspection combined with education of importers and travelers could prevent reintroduction of exotic plant diseases threatening U.S. crops once they are eliminated.

Quarantine can be effective in preventing long-distance spread of PPV within a region, state, or country.  If the disease is localized to a small area, it may be contained by local quarantines preventing movement of infected materials out of that area.  Such a quarantine was implemented October 21, 1999, in Adams County, Pennsylvania, by the Pennsylvania Department of Agriculture (PDA).  If implemented before movement of infected materials occurred, it will be effective in preventing spread of PPV.  The objective of the quarantine is to exclude PPV from entering other fruit-growing areas.  The inspection and testing of imported nursery material and plant breeding materials is the first line of defense against PPV.  It is also essential for commercial growers and nursery propagators to purchase only certified virus-free planting stock that has been tested and verified to be free of PPV, as well as other fruit viruses. 

Control through eradication.  Now that the virus is in the U.S., and since diseased trees cannot be cured, the next control strategy is to eliminate the virus-infected materials as quickly as possible before the virus spreads.  Intensive surveys are planned to identify the extent of PPV spread.

Because of quarantine concerns, the USDA has stepped in to work with the PDA and together, with advice from European scientists, they will develop a plan of action to eradicate the problem.  Although details of the eradication program are not available yet, preliminary discussions suggest that hundreds of acres of stone fruits including peaches, nectarines, plums and apricots will need to be destroyed.  Scouting and surveys to detect PPV will probably continue for several years to verify the effectiveness of the eradication program.  This will involve use of laboratory assays such as serological tests (ELISA), or nucleic acid probes for specific viral RNA sequences.  Once infected trees are identified, the cure is simple and administered with a bulldozer, chain saw, or other equipment to completely remove the tree, including roots.  The role of weeds or nearby wild prunus species in harboring the virus is not known.  If weeds and wild hosts are found to be important refuges for the virus, then eradication may be nearly impossible.

Control through protection of trees from aphid vectors.  Creating an insecticide barrier around healthy trees is not a practical means of preventing aphid transmission of PPV.  Control of this type of transmission is difficult because the aphids are so mobile and need to probe for only a few seconds to acquire or transmit the virus, leaving little time for insecticide effects.  Because aphids tend to lose the ability to transmit potyviruses if they land and test probe on a non-host plant before landing on the crop plant, decreased infection might result from surrounding Prunus with several rows of a non-host species such as apple.

Control through host plant resistance.  If PPV becomes established in the U.S., then plant breeding and genetic engineering may become important.  Unfortunately, little resistant germplasm has been identified in Prunus species. This means that few naturally occurring resistance genes are available for plant breeders to use in developing highly resistant fruit varieties.  Genetic engineering of resistant Prunus species may be possible by insertion of specific genes from PPV into the plant, but this approach has only been demonstrated experimentally thus far.

Now that the disease is in Pennsylvania, can they get rid of it?  At the moment there are many unanswered questions regarding how the virus arrived in Pennsylvania and exactly how far it has spread.  To date, PPV has not been found outside of two townships in Adams county.  These townships are under quarantine making it illegal to move Prunus trees or budwood from this area.  The USDA Animal and Plant Health Inspection Service (APHIS) and the PDA Bureau of Plant Industry (BPI) are working to prevent this disease from spreading to other parts of the country if at all possible.  The USDA has been successful in keeping PPV out of the country up until now since this is just the first report of PPV in North America, but with increased international travel and trade, this job becomes more difficult.  Officials are hopeful that because the strain of PPV found here tends to spread slowly in nature total eradication of PPV from Pennsylvania is feasible.  Presently, wild cherry, a common Prunus species growing in Eastern forests and fence rows, is also being tested to see if it is susceptible to PPV.  If wild cherry proves to be a host, PPV may be difficult to stop.  Within the quarantine area, home orchards and ornamental Prunus plantings are not excluded from PPV surveys and eradication, if found to be infected.

What are the implications of PPV for Kentucky?  Assuming the disease is eradicated from Pennsylvania, there would be little economic impact here.  Growers will probably not be able to obtain Prunus trees from nurseries located in the quarantine area.  If PPV is not stopped, then ultimately production of peaches and other stone fruits would not be profitable here until resistant trees are developed.  Use and maintenance of ornamental Prunus species in the landscape would also be compromised.  We should hope that the disease will be eradicated and that it will not appear here.

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