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| Landscape Alert newsletter | ||||
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| Vol. 21, No. 16, August 25, 2006 |
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In this issue |
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| Plum pox virus and the nursery industry
In July, the plum pox virus was detected in Michigan for the very first time. The detection was on a plum tree located at the Southwest Michigan Research and Extension Center located near Benton Harbor. |
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| History, biology and management of the plum pox virus Mira Bulatovic- Danilovich Bill Shane Ray Hammerschmidt The stone fruit disease plum pox was confirmed for the first time in Michigan in July 2006. Plum pox was first found in North America in Pennsylvania in September 1999. The Michigan Department of Agriculture (MDA) has participated in a national surveillance program for plum pox virus (PPV) since 2000. The discovery of PPV in southwestern Michigan was the result of this ongoing survey. The USDA's Animal Plant Health Inspection Service (APHIS) is also involved in the detection effort and determining the spread of plum pox in the United States. Symptoms The virus causes chlorotic rings on the fruit and leaves of infected plants. Fruit of infected plants may be of poorer quality and distorted. Symptoms vary depending on the plant type, variety, condition of the plant, time of year, and the length of time the plant has been infected. Many trees may not show symptoms for the first few years following first infection. Symptoms on mature peach fruit consist of chlorotic rings or line patterns. Apricot and plum fruit will often be bumpy. European plum varieties can develop red discolored areas and drop prematurely whereas Japanese plums show ring spot symptoms. The D strain can cause severe necrotic spots on leaves of some plum varieties; peaches may have chlorotic spots and streaks, and apricot leaves typically show only mild symptoms. Leaf symptoms are more easily seen in spring. PPV symptoms are sometimes difficult to distinguish from other diseases and may be confused with rusty spot of peaches and nectarines as well as insect-related problems such as damage from thrips, white apple leafhopper and San Jose scale. History Plum pox, also called Sharka, is considered one of the most serious virus diseases of stone fruit in Europe. The cradle of the disease is southeastern Europe. Early reports indicate Macedonia (part of former Yugoslavia) as the place where the first disease symptoms were observed as early as 1910. Growers in Bulgaria started reporting seeing pox on the plums during 1915-18. Bulgarian researcher Atanasoff was the first to write about the disease and its virology in his report ‘Sarka po slivite’ (Pox of Plums) in 1932. Since then, the disease spread slowly until after World War II. By the mid 1980's, it had spread to most of Europe with the exception of Netherlands, Belgium and Switzerland where it was presumably eradicated. By the late 1980’s it had spread to Cyprus, Egypt, Syria and India. In Europe, there are some 100 million trees infected presenting a serious economic problem. Susceptible varieties can have up to 100 percent crop loss due to diminished quality or massive fruit drop about 10 days prior to harvest. In 1992, it was found in Chile in the Western Hemisphere. Plum pox was first found in North America in Pennsylvania in October 1999. The USDA launched an extensive survey and eradication effort that eventually stretched over three counties. Plum pox was discovered in eastern Ontario and Nova Scotia in 2000, and western New York in early July 2006. PPV has apparently been eradicated from Nova Scotia. The USDA has pursued a vigorous policy of detection and eradication in the United States. What is plum pox? Plum pox is caused by a virus from the genus Potyvirus, one of the largest groups of plant viruses. It is the only known potyvirus infecting stone fruit Prunus sp. Four strains of PPV have been identified in the world. These strains are: PPV-M, PPV-EA, PPV-C and PPV-D. The most prevalent strain in central, eastern and southern Europe is strain M that infects peaches, plums and apricots. It is easily transmitted by aphids and, in certain varieties, by seed as well. It is considered to be the epidemic strain of the virus. Strain EA is known to exist only in northern Africa. It will infect apricots. Strain C is probably the most devastating strain due to the wide range of species that can be infected. It is known as “cherry strain” since the natural host range includes sweet and sour cherries. It was isolated from the sour cherry trees in Moldova. It could be very easily transmitted onto other Prunus sp. by inoculation. It was wide spread in central and eastern Europe. PPV-D strain is widely spread throughout western Europe. This is the only strain found in the Western Hemisphere, first in Chile, then in Pennsylvania and consequently in Canada, New York and Michigan. This Dideron or “D” strain of the virus infects peach, nectarine, plum, and apricots but not cherries. Research has shown that it is not seed-transmitted. The D strain is less aggressive than the M and EA strains. Numerous cultivated and weed annual plants can become artificially infected with PPV, but never have been shown to be a risk to stone fruit. Extensive surveys in Pennsylvania in areas adjacent to PPV-infected orchards provided no evidence that PPV has moved into wild species. Biology Infected peach, nectarine, plum and apricot trees are the primary source of PPV inoculum. The virus is spread to new areas by moving uncertified infected plant material through budding, grafting and transplanting, and by migrating aphids. Aphids are effective for spreading PPV within a tree and to adjacent trees. Aphids have several generations per year and have winged forms for movement from tree to tree. Spread by aphids over long distances is less common. Natural barriers such as hills and woods help to restrict spread. Several aphid species can serve as carriers for PPV. Among the most important species are the green peach aphid (Myzus persicae), leaf curling plum aphid (Brachycaudus helichrysi), peach leafroll aphid (Myzus varians), damson-hop aphid (Myzus humili), thistle aphid that overwinters on plums (Brachycaudus cardui) and the spirea aphid (Aphis spiraecola) as well as many less commonly found in commercial stone fruit orchards. Aphids common in stone fruit orchards generally have two to three generations per year and tend to be most abundant on lush foliage shortly after shuck split, especially during cool and wet springs. It is important to realize that some aphids such as cherry aphid (Myzus cerasi), mealy plum aphid (Hyalapterus pruni), and non-aphid arthropods such asleafhoppers (Edwardsonia plebei), lecanium scale (Lecanium corni) andplant bug (Lygus pratensis) are not capable of transmitting the virus. Aphids obtain the virus while probing and feeding on infected plant tissue with a straw-like stylet for sucking up plant sap. The highest concentration of the virus is found in the cell sap of the leaf epidermal cells. When the aphid penetrates with its stylet into one of those cells, the virus is pulled into the stylet where it remains. Plum pox is a non-persistent virus, meaning that the aphid retains the virus in its mouthparts and foregut until its next feeding probe, usually less than one hour, and rarely as long as several hours. The virus is transmitted as soon as the aphid spears the cell with its stylet containing sufficient amount of virus obtained by the previous probing. An important point is that the virus does not circulate within the aphid, and it does not replicate itself while in the aphid’s body. Infections spread slowly cell-to-cell within plant tissue from the point of initial aphid feeding. Gradually, the infection spreads throughout the entire tree, especially if aphids continue to feed on the tree. When the disease is first introduced to an orchard, the spread from tree-to-tree is slow, because of the low probability that a feeding aphid will encounter the virus. The speed of PPV spread in an orchard increases greatly as the number of aphids and number of infected trees increases. Management Management strategies of plum pox are aimed primarily at preventing introduction by use of virus-tested clean nursery stock. Once detected, strict quarantine, eradication and ongoing surveys are the only useful strategies because a tree, once infected, will never be free of the disease. Insecticide management strategies that keep aphid populations low may help to slow PPV movement in areas where PPV is rare, but may not be a good idea in some situations. Insecticide treatment can sometimes cause winged forms of the aphids to leave treated areas, taking the virus with them to infect new hosts. References Atanasoff, D. (1932). J. Univ. of Sofia Agr. Faculty 11: 49.Brunt ,A.A. et al. (1996). Plant Viruses Online. Jordović,M. (1965). Zastita Bilja. 85-88: 353-355. Jordović,M. (1967). Zastita Bilja. 96-97: 332-336. Jordović,M. (1968). Zastita Bilja. 100-101: 273-277. Jordović,M. (1969). Zastita Bilja. 104: 123-130. Jordović,M., Ranković, M and Festić,H. (1970). Zastita Bilja. Ann. De Pathologie.71-72: 179-184. Jordović,M, and Ranković,M. (1972). J.of Yugoslav Pomology. 21-22: 797-802. Levy,L., Demsteegt, V., Scorza, R. and Kölber, M. (2000). APSnet. 3/1-31. Nemeth, M. and Kolber,M. (1983). Acta Horticulturae. 130:293. Ranković, M. and Jordović, M. (1970). Zastita Bilja. 21:109: 195-199. |
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| List of known hosts of plum pox
Thus far, the only ornamental plant found to be infected naturally is P. glandulosa from one homeowner site in Pennsylvania. Other hosts Artificially infected hosts (Plants artificially infected in a lab that may be less susceptible in nature.) |
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| Michigan’s plum pox virus survey and restrictions Robin Rosenbaum Mike Hansen The Michigan Department of Agriculture has been surveying for plum pox virus (PPV) since 2000 as part of the Cooperative Agricultural Pest Survey (CAPS) program. CAPS is a combined effort by federal and state agricultural organizations to survey, detect and monitor agricultural crop pests and biological control agents. Survey for PPV was initiated in response to the identification of PPV in Pennsylvania in October 1999. Since 2000, the MDA has conducted PPV surveys every summer including a CAPS-funded survey in 2006. In July 2006, PPV was detected in a plum tree sampled at the Southwest Michigan Research and Extension Center (SWMREC) in Benton Harbor, Michigan. On August 11, the USDA laboratory in Beltsville, Maryland confirmed that the sample was positive for the "D" strain of the plum pox virus. MDA staff immediately began to sample 100 percent of the non-cherry Prunus trees at SWMREC and will continue to identify and survey all non-cherry Prunus trees within a radius of 5 miles of the positive tree. The survey will include commercial orchards, homeowner orchards, abandoned trees and landscape plants known to be susceptible to this strain of PPV. MDA's sampling effort will be supported by the USDA, Animal and Plant Health Inspection Service (APHIS) staff from the USDA's Plum Pox Survey Office as well as USDA inspectors from throughout the United States. It’s estimated that 150,000 trees will need to be sampled as part of this delimiting survey. If an additional positive tree is found, the survey will be adjusted accordingly to provide a 5-mile buffer around each infected tree. The USDA has issued Emergency Action Notifications (EAN’s) to all landowners with commercial orchards identified within 1 mile of the infected tree. Additional EAN’s will be issued to landowners with PPV-susceptible plants. The EAN provides notice that a landowner may not move propagative materials from PPV susceptible plants off of their properties. The MDA will soon initiate a quarantine of the same geographical area in accordance with Michigan’s laws. The restrictions in the quarantine will parallel those issued in the EAN’s. Neither the EAN nor the MDA quarantine will hinder the harvest and sale of fruit in this area since PPV is not transmitted through the fruit or the seed. The plum pox virus poses no human health risk. |
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| Warning: Yews are extremely toxic to livestock Steven Gower, MSU Diagnostic Services Wendy Osman, Michigan Department of Agriculture Earlier this season in St. Joseph County, a farmer found out the hard way that yews (Taxus spp.) are extremely toxic to cattle. A few overgrown bushes were removed from a landscape one afternoon and pulled into a pasture area. The intention was to burn them once all were removed and dried. The next morning, all 10 cattle in the pasture were dead. Plant material submitted to the lab was identified as Japanese yew (Taxus cuspidata). Taxus is the genus that includes numerous forms of evergreen shrubs or trees used in the landscape. Common species include Canada yew (T. canadensis), English yew (T. baccata), Japanese yew (T. cuspidata) and Anglojap yew (Taxus x media). Cultivars of the Anglojap yew, a cross between T. cuspidata and T. baccata, are some of the most encountered at garden stores. Yews have linear, sharp-pointed, approximately one-inch long, needle-like leaves that are dark green above and yellow green below. Leaves are spirally arranged but generally lie flat on the stem. Flowers are inconspicuous with mildly attractive, red, berry-like fruit. (view photo) All parts of yews, except the red, fleshy part around the seed, are extremely toxic to cattle and other livestock. The primary toxin is taxine, which is a mixture of alkaloids that act upon the heart and usually result in death of the animal. Uprooted bushes and clippings should never be placed near livestock. Yew is generally not highly palatable to livestock. However, animals are more likely to graze on tender new growth or clippings discarded in a pasture. As little as a mouthful to 1 lb. of yew clippings is sufficient to kill a 1000 lb. animal in only a few minutes. |
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| Attention nursery and Christmas tree growers: MAEAP is for you
Nursery and Christmas tree growers can get a first-hand look at how their operations can become environmentally certified through the statewide Michigan Agriculture Environmental Assurance Program (MAEAP) by attending an on-farm field day in September. Field days are scheduled forSeptember 15, 10:30 AM to 1:30 PM at Trim Pines Farm, 4357 E. Baldwin Road, Holly. (Qualifies for 1 pesticide applicator credit for commercial or private core) September 22, 1:30 PM to 3:30 PM at Ottawa-Kent Perennial Gardens, 2089 Hayes St., Marne. (Pesticide applicator credits applied for) Nurseries hosting the field days were used as test sites to help develop a new environmental assurance program for nurseries and Christmas tree operations. Nursery owners partnered with teams of specialists from MSU Extension, MDA and county conservation districts to conduct voluntary environmental risk assessments of the nursery and all associated production field. There is no charge to attend the field days, which are sponsored by Project GREEEN, MSU Extension, the Michigan Groundwater Stewardship Program and MAEAP. Advanced registration is appreciated. For the September 15 field day, contact Tom Wert (989-723-8263 x 3). For the September 22 field day, contact Tom Dudek at (616-846-8250). MAEAP is a three-phase program built on scientifically supported standards that allow farms to address environmental concerns while remaining economically viable and sustainable. Phase one is education, which consists of introducing farmers to MAEAP and updating them on new and emerging environmental regulations. Phase two, on-farm risk assessment, focuses on evaluating on-farm environmental risks and devising farm-specific corrective and economically viable solutions. The final phase, third-party verification, is performed by the MDA. MAEAP is an innovative, proactive program that helps farms of all sizes and commodities voluntarily prevent or minimize pollution risks. It teaches producers how to identify and prevent environmental risks and how to apply effective land stewardship practices that comply with state and federal environmental regulations. MAEAP was developed by a coalition of agriculture producers, commodity groups, state and federal agencies, and conservation and environmental groups. To learn more about MAEAP and how your farm can become MAEAP certified, visit www.maeap.org. |
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| Time to seed Kevin Frank Crop & Soil Sciences Now is the optimal time to look at reseeding or sodding any turfgrass areas lost to the heat, humidity and disease that sporadically plagued turfgrass managers this summer. Daytime high temperatures have started to dip and hopefully the pattern of recent rainfall will continue in the coming weeks to aid any new establishment projects. This article will focus on tips for renovating/reseeding damaged turf areas. Future Landscape Alert articles will focus on fall turfgrass management practices to ensure a successful 2007 including core aeration, broadleaf weed control and fall fertilization (we’ll touch on fall fertility in this issue). Reseeding damaged areas The first decision you need to make is whether to spot seed the bare areas or if the whole area needs to be reseeded. This decision is basically one of practicality. Many turf areas made it through this summer relatively unscathed and turf death tended to be scattered. If the turf was simply thinned or there are small patches of dead grass, a fall fertilizer application and favorable weather conditions should facilitate the existing turfgrass stand to recover and fill in those bare spots. If the damaged areas are large, for example lawns that essentially have only sporadic green turf plants, then overseeding is going to be necessary to restore the area to turf. Tools to renovate There are a number of different methods to ensure that reseeding efforts are successful. First and foremost, you must ensure you get good seed to soil contact, ie. seed sitting on top of the soil or simply broadcast spread onto the turf without creating any holes or slits will not be successful. There are several options for creating the holes/slits to ensure seed to soil contact. Machines such as core aerifiers, power rakes, slit seeders or even hand raking small areas should get the job done. For established areas that may just need a light overseeding to improve density, a core aerifier is a viable option. It’ll also give you the benefit of improving soil aeration, which is critical for many sites that have compacted soils or high clay content. For areas that need lots of help, I really like slit seeders; these machines operate by creating a slice in the soil that the seed falls directly into and ensures the seed gets to where it needs to be. Many lawn care companies offer this service, or if you’re a do it yourselfer, these devices can usually be rented at the local hardware or rental store. Species, seeding depth and rates Making sure you have the correct species and cultivar, especially if you are overseeding an existing lawn, is a critical step to ensure satisfaction. One common frustration many property owners have after overseeding is that the newly seeded turf has a drastically different color/appearance than the existing turf stand. To avoid this problem I would suggest you do your homework to try and find out if you know the specific species and cultivar that was originally established. In most areas, if you’re not sure of the turfgrass species on the lawn, odds are its Kentucky bluegrass. Therefore, obviously you should select Kentucky bluegrass cultivars to reseed the turf. If, however, you are completely renovating an area and are looking for something a little different that might be able to withstand drought conditions better, I would give tall fescue a try. Look for catch words such as “Turf type” or “Improved” when selecting tall fescue cultivars. I would avoid the standard Kentucky 31 (K-31) tall fescue. It has its use in minimal maintenance landscapes or parks, but in most lawns it would look ugly due to its wide leaf blade. Turf type tall fescue is now being mixed with Kentucky bluegrass and is becoming more widely available to consumers, this might be a good option for folks who want to experiment a little and see what they think. Please see our fact sheet on Purchasing Quality Turfgrass Seed: Read the Label at www.turf.msu.edu under the homeowner section for more tips on purchasing seed. Overseeding rates for Kentucky bluegrass are 1.5-2 lbs./1000 sq. ft. For mixtures containing perennial ryegrass or fine fescues, rates should be increased to 3-5 lbs./1000 sq. ft. For tall fescue the seeding, not overseeding, rate is 6-8 lbs./1000 sq. ft. Strive to incorporate the seed to a depth of about ¼ inch. Deeper planting depths may result in some of the seed not making it to the surface. Consider mulch for moisture retention Spreading a light mulch cover on top of the newly seeded area will help the soil retain moisture and keep the seedlings from drying out. The recommended rate for using straw mulch is 1 bale of straw/1000 sq. ft.. Be careful with how much mulch you apply, you don’t want to end up smothering those young seedlings. Apply enough so you can still see about 1/3 to ½ of the soil underneath. There are also numerous, more sophisticated mulch products that are easier to spread than straw and expand with moisture to cover the soil. Even something as simple as turf clippings can be used as mulch; once again, don’t spread them too thick over the area. Fertilizer, irrigation and herbicides At the time of seeding, apply a starter fertilizer at a rate of 1 lb. N/1000 sq. ft. to help those young seedlings get established. Make sure to keep the seeded area moist throughout establishment. In many cases, this may require watering several times a day. A good mulch cover will help the area stay moist so the site may be watered less frequently. Water lightly when irrigating – there is no need to see water puddling or running off the site. To be safe, avoid applying all herbicides this fall, i.e. no weed and feed products. Young seedlings don’t tolerate herbicides very well and the guideline is usually to wait three "real" mowings before applying any herbicides or in some cases at least 60 days. By "real" mowings, I mean you’re actually cutting significant grass, not just running over the area to trim down any weeds. Finally, don’t be afraid to get out there and mow the area. It’s always challenging to set absolute guidelines when talking about when you should start mowing new seedings. My recommendation, don’t wait until the seedlings are so tall they’re starting to fall over. If you typically mow your lawn at 3 inches – start mowing the newly established areas when the seedlings get to or slightly exceed the 3 inch height. Mowing turf helps it spread laterally and fill the area, so start those engines as soon as you start to see the grass creep up to your established cutting height. Hopefully, these tips will help you reclaim those bare areas and get the turf off the blocks quickly this spring. |
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| Fall fertilizer application Kevin Frank Crop & Soil Sciences Labor Day is one of the holidays that comprise the “Holiday” fertilization calendar. Fertilizing turf around the Labor Day holiday will help the turf recover from summer stress. A September fertilizer application will help by stimulating tillering and rhizome growth (lateral spread of turf) that help the turf fill in bare areas. Consider applying a slow release fertilizer that will “feed” the turf through September and October. There are numerous fertilizers that are considered slow release including: natural organics, sulfur or polymer coated ureas, methylene ureas, and IBDU. |
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| Look for Douglas fir needle midge Jill O’Donnell, Christmas Tree ICM Agent Deborah McCullough, Entomology The swollen, yellow bands caused by Douglas fir needle midge can be seen be easily now on Douglas fir needles. Inside these needles, you can find the midge larvae, but you will need a hand lens. (See images at: http://www.ipm.msu.edu/CAT03_land/L09-05-03.twiggall2.htm) As the season progresses, these areas will eventually turn brown. In the fall, the larvae complete development and will drop from the needles and enter the soil beneath the infested tree. Next spring these larvae pupate and adult midges emerge as buds are expanding. This is when you will need to time your control efforts. Note: These symptoms can be confused with Rhabdocline needle cast and Cooley spruce gall adelgid damage. |
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| Start scouting for spruce spider mites Jill O’Donnell, Christmas Tree ICM Agent Deborah McCullough, Entomology Growers will want to begin scouting again for spruce spider mites. Spruce spider mite is a “cool season mite,” and we sometimes see populations build in the fall of the year. Pay particular attention to Fraser and spruce trees you plan to harvest this year. Check for mite activity by closely inspecting the older needles near the tree stem or by sharply rapping shoots over white cardboard or a notebook with white paper. |
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| Weather news Jeff Andresen Agricultural Meteorology Geography The unsettled weather pattern of the past few days will continue into at least the first half of the weekend as a frontal boundary persists across the region. Showers and thunderstorms will be possible again across the state Friday (August 25) into Saturday with a continuing chance in eastern sections of the state through Sunday. Best chances for rainfall during the next couple of days will be over eastern sections of the state where rainfall totals are forecast to reach the 0.25- to 0.5-inch category with a few 1 inch totals possible. Lighter amounts are expected elsewhere. Temperatures during the next few days will generally average near climatological normals with highs generally ranging from the low 70s north to the low to mid-80s south with lows falling to the low 50s north to near 60 south. Fair, dry, and cooler weather is expected to return by the end of the upcoming weekend into early next week with another Canadian-origin high pressure system. Based on current forecast guidance, dry conditions are expected to continue through the next work week and into next weekend. Medium range forecast guidance calls for a generally west to east zonal pattern over much of North America with weak troughing features over the Pacific northwest and northeastern United States and a broad, relatively flat ridge over the middle of the continent. The official NOAA Climate Prediction Center 6-10 day and 8-14 day outlooks (covering August 30 through September 3 and September 1-6) both call for above normal temperatures statewide. Both outlooks also call for the upper air ridge to effectively suppress precipitation over much of the Midwest region, with precipitation totals forecast to remain at below normal levels during both time frames. |
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