Pest Alert

May 15, 1999

Welcome to the second issue of the 1999 growing season. Spring is an active time of growth for plants and pests. Weather is playing a big role in that it has been unusually wet and cool for this time of year. While the cool conditions slow down pest development, the wet conditions foster a number of foliage diseases.

Categories for topics are as follows, H - Herbaceous plants and flowers, W - Woody Plants, V - Vegetables, F - Fruit and L - Lawns.

DISEASES

W,F APPLE SCAB activity is probably on the rise in many areas of the state thanks to the recent rains and humid weather conditions. Continue applications of fungicides for control of this fungal disease. Fungicides such as Captan and Benlate should be effective (read label for directions).

W ASH ANTHRACNOSE is being reported as very heavy in many areas of the state, particularly in the north central part of the state. Treatment is rarely necessary and may actually be too late already this season as trees have fully leafed out in most areas. However, the best treatment is to apply Captan, beginning just after bud-break and continue at 10-day intervals as long as cool (below 55F), moist conditions persist.  Typical symptoms include small brown dots on infected foliage and leaf drop, often followed by larger irregular dark blotches on leaves.

W ASH RUSTis beginning to show up on ash in some areas of the state. Symptoms are yellow to yellow orange spots on the upper surface of the leaves. Within several weeks, these affected leaves will become distorted and develop a sharp bend at the petiole. Ash rust can be treated with Bordeaux mix applied when the leaves are developing. Again, once the symptoms are observed, it is too late for effective control.

W,F BLACK KNOT cankers continue to develop and will be releasing spores soon. Prune out new swellings on the branches to reduce the spread of this disease.

F,W CEDAR-APPLE (HAWTHORN) RUST is showing up on eastern redcedar and Rocky Mountain juniper across the state with recent rains. Affected plants are sometimes completely covered with orange, gelatinous round masses on the tips of branches. Some young plants have had branches break under the heavy load of these fruiting bodies. These fruiting bodies are releasing spores that will infect susceptible apples, crabapples, hawthorns and juneberries. Symptoms of the rust on these trees include bright yellow to orange spots on the leaves. This summer these infected deciduous hosts will produce fruiting bodies that will release spores that infect the junipers. These evergreens can be protected with an application of chlorothalonil when spores are released later this summer. Do not spray the junipers now. It will not eliminate the fruiting bodies. Galls may be pruned off the branches to reduce the likelihood of having neighboring apple trees infected.

Cedar Hawthorn rust, a related disease, can infect apple fruit causing it to become malformed. Preventive fungicides include Ferbam, Thiram and Maneb which will also control apple scab. While apple trees may be sprayed to protect them from cedar-apple rust, it is often not necessary since the leaf spotting typically does not significantly harm the tree or reduce fruit yield.

W CROWN RUST ON BUCKTHORN is common in many areas. It causes bright orange swollen spots on the leaves, petioles and green stems of buckthorns. Buckthorns are the alternate host for oat and rye crown rust but the rust may attack a wide range of other grasses. No treatment is recommended.

W DOTHISTROMA BLIGHT OF PINES: Ponderosa and Scots pine in some parts of the state have been developing symptoms of Dothistroma blight over recent days. Ponderosa and Austrian pine are very susceptible to this disease. Dothistroma is a needle disease that causes dark green to brown banding on the needles and browning of the needles from the banded area outward. As a result you wind up with needles that are half to three-quarters brown from the tip inward. There should also be discrete spots present on the needles. It is easy to confuse this disease with winter injury so bear in mind when the damage appears and use a hand lens to look for dark structures erupting through the epidermis of the needles in association with the spots and bands, especially near lower end of the dead area. Most infections occur during wet periods in the mid-summer. Young needles are immune to infection, but they become susceptible to infection after a month or so. Dothistroma blight can be treated with fungicides, but the timing is different than would normally be used with spruce needle casts. Apply a protectant fungicide such as chlorothalonil, Bordeaux mixture or choose from several copper fungicides that are labeled. Spray in the spring before bud break to protect the older needles and a second time in the mid summer to protect the new needles. The first application is to protect older needles. The second application protects the new growth. This disease is typically not as damaging as needle diseases on spruce, but it can cause substantial losses if left uncontrolled. Control is very successful on ornamental plantings following two years of fungicide applications.

W,F FIRE BLIGHT has been quite severe the last few years on apples, crabapples, cotoneaster and mountain ash across the state. This bacterial disease is carried from tree to tree by insects. The cankers on infected trees will soon begin oozing a milky to amber-colored exudate that contains the bacteria. This sticky material attracts insects that carry the bacteria to other trees. The bacteria enter the host through the blossoms, leaves or wounds. Since most apples are now in bloom, this disease may cause numerous blossom infections this year. Growers should be on the watch for signs of dying spurs and small twigs over the next month. (Photo of Mountain Ash twig with a blossom infection.)  Pruning equipment should be dipped in a solution of 1 part household bleach and 9 parts water between cuts. Avoid heavy applications of nitrogen to infected trees.

L MUSHROOMS have made a rapid appearance in many home lawns following recent rains. They are actually the fruiting structures of a much larger fungus body growing beneath the soil surface. The most common mushrooms, seen growing in lawns, are the inky caps, which grow up in large clumps in the lawn, usually close to an old stump and fairy ring mushrooms. These mushrooms are usually smaller than the shaggy cap mushrooms and they generally grow up as single mushrooms but are arranged in the shape of a ring. They are also growing on organic matter in the soil but are generally not closely associated with an old stump.

There is no easy control for either of these mushrooms short of digging up the lawn to remove the organic materials they are growing on. The best solution is to just rake them out when they appear or simply ignore them.

W,F RHIZOSPHAERA NEEDLE CAST will soon be active and spreading to the new growth on blue spruce. Best treatment is to use Chlorothalonil or Bravo, apply it now, when the new shoots are half elongated (about 1/2 to 2" long) and again when needles have fully elongated (about 3 weeks later). Repeat this treatment for two consecutive years to control this disease. This disease has been very serious the last few years and is expected to continue to be a problem as long as we continue to have wet spring conditions.   Photo of spruce twigs showing black fruiting bodies coming through stomata on needles.  Close up image of same needles.

 INSECTS

W APHIDS are now infesting viburnums in the central part of our state. The infested plants can be identified by their curled and distorted leaves. Under closer examination, small soft-bodied insects - aphids - can be found sucking fluid from the leaves. Since the insects are protected by the curled leaves most contact insecticides will not provide adequate control. The best control is the use of a systemic insecticide such as orthene.

W ASH BARK BEETLE ADULTS are now flying throughout the state. The adults are small, less than 8mm, and are gray in color. These small beetles will be attacking stressed green ash trees in urban and shelterbelt plantings. Trees that have been attacked are easily identified by the numerous galleries beneath the bark. Trees that are attacked by ash bark beetle are generally stressed, usually due to continuous flooding. Spray the trunks and lower branches of infested trees with Dursban or Lindane now and repeat in 2 weeks.

V CUCUMBER BEETLES are probably waiting for your young cucurbit seedlings to emerge. These small voracious eaters can totally consume young seedlings or they may just feed enough to inoculate the plants with bacterial wilt. In either case you will want to be on the watch for these small black and yellow striped beetles and use protective measures to reduce their damage. Floating row covers work very well to prevent the beetles from getting to the young plants. Also, treatment with Sevin or Rotonone containing products should be effective as well.

V FLEA BEETLES may already be feeding on your young vegetable seedlings. They especially like radishes and eggplant where they feed making tiny holes in the leaves. The tiny beetles are often difficult to see since they are quick moving and rather elusive. Protective treatments with Sevin, malathion or Rotonone should be effective.

W HONEYSUCKLE APHIDS will soon be actively feeding on shrubs all over the state. These are the insects responsible for the malformed "witches broom" type appearance of the new shoots. A treatment or Orthene should be applied once the new growth emerges in early May and repeated at 6-week intervals.

W MAPLE BLADDER GALL MITE calls are coming in across the state. Every year people become alarmed at finding these small, multicolored bumps on the upper leaf surface of their silver maple trees. These bumps are due to feeding by the mite and it’s many offspring. Control is not effective at this time as the galls have already formed and will not go away. Control is rarely warranted at any time as even trees with every leaf infested with the galls show no injury due to the feeding or gall formation.

W MOUNTAIN PINE BEETLES are still in the larval stage but should be pupating in the next couple of weeks. Adult emergence will probably be in early July though this can change if the weather warms up. Trees that were attacked last year can be identified by the pitch masses along the lower 10 or 15 feet of the trunk and sawdust at the base of the tree. These infested trees should be removed and burned before June 15 to prevent the emergence of new adults. Merely felling or felling and debarking is not effective at this late stage of development. (Photo of area devastated by Mountain Pine Beetle.)

W PINE ENGRAVER BEETLES, otherwise known as Ips, are now emerging in the Black Hills. These bark beetles generally attack stressed pine trees and are a persistent problem in the Black Hills. There are at least two and possibly three generations per year. They can be controlled by a single application of Sevin XLR made now. This application should provide a season of control. (Photo of galleries beneath the bark.)

W SCALE CRAWLERS will be emerging soon. The crawlers are the newly hatched nymphs of the adult scale insects. Crawlers are quite easy to control since they have not yet developed the tough waxy body covering characteristic of the adults. Pine needle scale and oyster shell scale are the two most common scale insects found in South Dakota. While a dormant oil spray can do a pretty good job of controlling scale, it is probably too late to apply now since most shrubs and trees have already leafed out. Check for crawler emergence by doing a tap test as is done with spider mites. Once the crawlers emerge use a horticultural oil (Sunspray) or insecticidal soaps to control them. Malathion, diazinon or orthene may also be used. In severe infestation on deciduous shrubs, heavily infested branches may need to be pruned out to help the plant recover.

WEEDS

L APPLY PREEMERGENCE TURF HERBICIDES NOW. Yearly infestations of annual grasses such as crabgrass, goosegrass, and foxtail in established turf are best controlled preemergence before weed seeds germinate. Crabgrass seed germinates when soil temperatures of 50 to 55F are sustained for several days. Germination of goosegrass and foxtail generally follows crabgrass by about two weeks. Products using pendimethalin, dithiopyr, prodiamine, oxadiazon, or trifluralin + benefin are effective. Be sure to follow preemergence applications with about 1 inch of irrigation (or rainfall) in order to move the herbicide into the top 1/2 inch of soil where weed seeds germinate. If you purchased a postemergence product, wait uyntil the crabgrass is emerged. That may be in 1 to 3 weeks,, depending on the weather.

L DANDELION SEASON has been extended by the cool wet weather. Spring treatments will stop further bloom on older plants – the real benefit may be in controlling the heavy flush of new plants just getting started. Use caution to avoid damage to sensitive plants. Touch up missed dandelions by cutting the root below the surface (not really "digging") or use a "chemical cane" that dispenses weed solution directly onto the plant. These are "no-drift" methods.

L GROUNDIVY AND HENBIT. Not everything with a small purple flower in the lawn is groundivy or creeping charlie. It could be henbit. We have more samples and inquiries about henbit this season than past years combined! It grows as a winter or spring annual. Stems are square (mint family). Leaves are lobed and opposite. They are clasping at the top and have short petioles lower. It grows more upright than groundivy.

L,W TRIFLURALIN alone ( such as Preen) is not labeled for use in lawns. Labeling includes many perennials and ornamentals found in beds and borders. It will not control emerged weeds and is not effective for perennials.

OTHER NOTES

W,F,H FROST DAMAGE is suspected on a number of samples that have come in from the south-central part of the state. The most commonly affected plants appear to be lilac, Norway maple, pear, elm and other sensitive plants. Most samples exhibit severe blackening and collapse of the new growth. Prognosis is difficult at this point but new growth should emerge later this spring and the plants will survive but that may not always be the case. There is no treatment except good normal care. Later in the season, dead shoots may be pruned out to improve the appearance of the plant.

L SPRING FERTILIZATION. Mid-May is the ideal time to apply the initial fertilizer application of the year for home lawns. Application at this time follows the end of the natural early spring shoot growth surge following winter dormancy and helps to prepare the lawn for dealing with the stresses of summer. Use a complete fertilizer, which includes nitrogen, phosphorus and potassium at the rate of 1 lb. nitrogen per thousand square feet. One-fourth to one-third of the product should be in a slow release form, such as sulfur coated urea, ureaformaldehyde, isobutylidine diurea, polymer coated, or natural organic. Uniformity is best achieved by applying the product in 2 perpendicular directions at one-half the rate using either a broadcast or drop spreader. If a preemergence herbicide/fertilizer product was applied earlier in the spring, do not fertilize again at this time.

L SUMMER MOWING HEIGHT. Reduce summer lawn stress by maintaining the proper mowing height for home lawns. A consistent mowing height of 3 to 4 inches will help to maintain a deeper, more extensive root system during summer when cool-season turfgrasses lose much of their roots due to high temperatures. A reduced root system does not allow adequate water uptake to meet transpirational needs, causing stress to the turfgrass. Since turfgrasses are more susceptible to diseases and insect pests when under stress, maintaining a proper mowing height can help reduce or prevent pest problems later this summer.

W,F NEWLY PLANTED TREES, SHRUBS, BEDDING PLANTS AND VEGETABLES will need some extra attention in areas of dry soil conditions. Be certain to provide adequate water to help these plants get established in their new site. Water thoroughly to saturate the root zone around these plants, and extend the watering out several feet from the trunk to encourage new roots to grow out from the old root ball. Do not keep the soil continuously wet however. Roots need oxygen to grow too so allow the soil to dry a few days before watering again.

L PERSISTENT, WET CONDITIONS in many areas of the state have accelerated growth of turfgrass in lawns. Saturated conditions and frequent rainfall may interrupt your normal mowing schedule. Under normal conditions, no more than 1/3 of the shoot length should be removed at any one mowing, and clippings should be returned to the lawn. If you’re unable to keep up with proper mowing frequency, excessive amounts of clippings may be deposited on your lawn. Be sure to remove heavy clipping deposits from your lawn. Failure to remove these clippings may reduce the amount of sunlight available to the turfgrass, and may also initiate disease problems.

W PREVENTING FRUITING of crabapples and other ornamentals may be accomplished at this time of year. There are several products that can be used. They vary on effectiveness and availability. The most commonly available product is the insecticide Sevin. A 50% WP formulation can be applied at a rate of 3 T/gallon of water, two to three weeks after full bloom. DO NOT use Sevin on a tree while flowers are still present as it is very toxic to bees. Sevin will help to greatly reduce problems with messy fruit that fall from the tree and make a big mess on sidewalks, driveways etc.

Two other products are often more effective than Sevin. The first is sold under the trade names of Fruitone N and App-L-Set. These contain the plant growth regulator Napthaleneacetic Acid (NAA). NAA type products need to be applied at full bloom for best effectiveness. Another product that is effective if applied at full bloom is sold under the trade name of Florel Fruit Eliminator. The active ingredient is a chemical called ethephon that breaks down into another plant growth regulator - ethylene. The presence of ethylene causes the plant to abort its flowers and therefore greatly reduce the production of fruit. Typically products containing either NAA or ethephon will be more effective than the insecticide Sevin in reducing fruit set on the tree.

W 2,4-D DAMAGE is also appearing on a number of ornamental plants. The damage is similar to that caused by aphid, curling and distorted leaves, but in this case no insects can be found.  Do not prune out suckers at the base of your trees at this time. Generally homeowners start pruning out the suckers at the bases of their crabapples and other small flowering trees at this time, however it is not the best time to perform this practice. Suckers removed in the spring often come back very vigorously by mid-summer. Instead, homeowners should delay removing the suckers for a couple more weeks. Suckers removed in June usually do not come back with a much vigor or reappear as numerously. Suckers arise from the seedling rootstock of the flowering trees and should be annually removed to prevent them from overtaking the particular cultivar.