Pest Alert

May 25, 2000

Welcome to another issue of Pest Alert.  While it is still dry in some areas of the state, rain (or snow) has fallen on many places reducing the fear of drought but increasing the possibility of foliar diseases.  The warm weather throughout much of the state, whether wet or dry, has hastened the development of plants and their pests so problems may be appearing earlier than expected. 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.  Continue applications of fungicides for control of this fungal disease.   Fungicides such as Captan and Benlate should be effective (read label for directions).  Photo of apple scab infection on apple leaves.

W   ASH ANTHRACNOSE may begin to show up with recent rainy weather creating the proper environment for the disease to get established.  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.

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  Diplodia tip blight is appearing on Austrian pines throughout South Dakota.  The disease is also occurring on ponderosa pines and some Scots pines.  The symptoms are new shoots and needles turning brown and becoming stunted.  Trees that have been infested for several years often are disfigured from all the stunted tips.  Treatment for this disease is an application of Cleary 3336F (thiophanate) just as the buds are expanding (occurring now throughout the state) with two more applications spaced 10 days apart.  This treatment, the best available, is still only about 80 percent effective so it is impossible to eliminate the disease, only manage it.  Other treatments, such as lime sulfur, are less effective; generally providing less than 40 percent control.

H  IRIS LEAF SPOT infections are also severe this year causing the characteristic tan colored spots on the leaves.  Later, heavily infected leaves turn brown and die back from the tip.  The best control for this disease is prevention.  Thorough clean up of old leaves in the fall or spring before growth begins can significantly reduce new infections because the disease overwinters on infected leaves.  Heavily infected leaf tips may be cut off.  A protective spray of Captan or chlorothalonil will help reduce further infection.

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 two 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  Pine wilt.  Nebraska is beginning to experience an increase in losses of their Scots pines due to the pine wood nematode.  This nematode is transported to susceptible pines by sawyer beetles.  Once in the tree the damage the nematodes do to the sapwood, as well as the blue-stain fungus that is introduced at the same time, can cause a pine to yellow and die within a growing season or two.  The nematode is mostly a problem with introduced pines – Scots and Austrians – ponderosa and other native US pines are not as seriously affected.  We have had reports in years past of pinewood nematode in Scots pines in Lake Andes and Bennett County.  The nematode has been found in ponderosa pines near Ft Meade but it was not injuring the trees.

W  Dying river birches are a common concern in many East River locations.  The typical symptoms are the top one-third or one-half of the canopy failing to leaf out or the foliage that does develop is sparse.  The problem is not the bronze birch borer – the borer does not attack river birch – but winter injury.  River birch is at its northern limits in South Dakota and it is common to loose a number of trees each winter.  This winter losses were worse, not because the winter was extremely cold, but the fall was too dry.  The trees went into dormancy stressed from the lack of moisture.  This reduced their ability to achieve as deep of dormancy as possible and our cold winter temperatures, particularly the late winter, did the rest.  Avoid pruning any wood that appears dead but is still green beneath the bark, it may still leaf out this year.

INSECT CONTROL

W  APHIDS are a common sight on a wide array of ornamental trees and shrubs, especially in drier areas of the state.  They are feeding in large numbers on the new succulent shoots and leaves causing leaf and shoot malformation.  Treatment may be warranted on young or newly established trees and shrubs.  Best treatment would be to use Orthene since its systemic activity will be more effective at getting the aphids hiding in curled leaves.  Malathion, diazinon and Insecticidal Soaps could also be used.  Forceful jets of water can also remove many of the aphids from the foliage.

W  Ash borers will be soon flying through the state.  The best time to treat is about 10 days after the first sustained male catch in pheromone traps or about one week after Vanhouttee spirea begins to bloom.    A single application of Dursban (chlorpyrifos) should provide good control.  The adults appear wasp-like and lay their eggs on stressed or injured ash – “lawnmower blight” trees are excellent candidates as well as trees that have their trunks wrapped.  After the eggs hatch the larvae crawl on the bark surface for a short time then enter the tree to begin feeding in the inner bark.  The symptoms to an ash borer infestation are yellowing foliage, wilting foliage and branch dieback.

V   BEAN LEAF BEETLES.  These small beetles can be seen feeding on young bean plants in the garden.  They eat small 1/4" holes in the leaves.  Feeding can be rather heavy resulting in considerable loss of foliage.  Beans growing in gardens with a history of the problem or near old soybean fields are most likely to be attacked. Protective sprays or dusts of Sevin or Rotonone are quite effective.

W Bronze birch borer should be flying very soon.  Emergence is usually around the time buckeyes come into bloom. The dark coppery, torpedo-shaped adults can be found walking on the bark of trees during warm, sunny days.  The adults lay eggs near where the branch attaches to the trunk and the eggs – invisible to the eye – hatch in about 14 days.  The newly hatched larva immediately tunnels into the tree and begins feeding on the inner bark.  This feeding results in bumps forming along the trunk.  The feeding “cuts” the connection between the leaves and the roots so the roots starve.  As the roots die, there is less water carried to the top of the tree and the canopy begins to yellow and wilt.  Eventually the branches die back and usually within a couple of years the entire tree dies.  The adults can be treated with a bark application of Turcam (dycarb) in another week with the application being repeated in three or four weeks.   Trunk injections are also an excellent means of treating this pest.  However, injections should be delayed until after all the eggs hatch and the larvae are feeding, probably about the end of June or early July.  Keep in mind, however, that bronze birch borer and the ash borer, do not generally attack healthy trees.  Trees that are overmature - 40 or more years for birch, 80 or more for ash – injured or suffering from moisture stress are the most susceptible.

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.

W  Hackberry nipple gall makers are out.  The adult psyllids look like very small cicadas.  They are laying eggs on the expanding leaves of hackberries throughout the state.  When the eggs hatch the nymphs begin feeding on the underside of the leaves and the leaf forms a bump or gall around the insect.  Once the gall has formed nothing short of nail clipper will remove them.  To prevent the galls from forming trees should be treated with Sevin (carbaryl) as the leaves form.  We do not recommend treatment, however, as the galls rarely harm the tree – even if all the leaves are covered.

W  Pine sawflies have hatched on mugo pines in southeastern South Dakota.  These insects appear as small “worms” or caterpillars but have eight pairs of prolegs. The small larvae can be found feeding in colonies along the needles. They also rear up in a characteristic “s”-shape when disturbed (scientist think this is to scare away birds).  The feeding damage from these sawflies rarely kills the trees as the feeding is on old, not new, needles, but it can disfigure the tree.  Recommended treatment is Sevin (cararyl) or Othene (acephate) when the sawfly larvae are observed.  Insecticidial soaps may also be effective.

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.  Check for crawler emergence by doing a tap test as is done with spider mites.  Once the crawlers emerge use an insecticide to control them.  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. Superior oils (Sun Spray) can be used after leaves emerge and is highly effective on the crawlers yet does not harm predatory and other beneficial insects. 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.  Also, check the plants for other types of site-related stresses like water, temperature, light or nutrient extremes, which may be contributing to poor plant health.  Remember, heavy scale infestations are often a symptom of other problems.

W TENT CATERPILLARS have been actively feeding on many trees and shrubs, especially plums, cherries and apples.  As the larvae continue to grow, they become more destructive as they are able to eat more and are more mobile.  Many of the larvae may be killed by simply crushing and destroying the web they have begun.  You could also use  carbaryl (Sevin), Malathion or Bacillus thuringiensis (B. t.) containing products. 

OTHER NOTES

W,F,H FROST DAMAGE continues to generate a number of samples.  Most samples exhibit tattered, browned foliage or the complete collapse of the new growth.  Leaves may have jagged holes in them from damage earlier in the season.  Prognosis is difficult at this point but new growth should emerge later this spring and the plants will likely recover.  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.

W,F,H HAIL AND STORM DAMAGE has been a common problem in some areas.  Herbaceous plants, in hardest hit areas, may have been completely broken down by the hail, while woody plants may have been completely defoliated.  Besides the defoliation, the countless wounds on the stems open the plants to infection by various disease pathogens that could cause further damage as the plants struggle to recover.  The herbaceous plants may start new growth from buds lower on the crown but annuals may not recover at all if no live buds were left on the plants.  Clean up damaged foliage and other debris then wait to see what sort of regrowth occurs.  If a shoot is going to regrow, new growth should be apparent in a few weeks.  If no growth develops in a month, the damaged shoots should be pruned out at their base.

Hail will strip leaves from trees and damage smaller stems with elongated wounds that may cause the branch to die or may open it up for infection by various canker diseases.  Many branches have been broken from trees and shrubs with the recent storms.  The best approach is to remove the stubs back at the main trunk or close to the ground.  This should help to improve the appearance of the plants and allow for the best chance at recovery.   Another common type of damage to trees is split co-dominant leaders, particularly in silver maples and green ash.  Many people have removed one of two split branches thinking that will solve the problem.  Unfortunately, the exposed wood on the remaining branch will be very susceptible to decay as well as being very weakly attached.  Instead of removing the branch at the split, the entire co-dominant branch should be removed at the next lowest crotch.

W  POLLEN IS BEING PRODUCED BY PINES at this time and it is especially noticeable on mugo pines.  If you shake the branch, a cloud of yellowish pollen will be produced.  Several homeowners have called about "the dust coming off their pines."  This pollen production is of no concern even to “hayfever” sufferers.  The pollen of pines and other gymnosperms is not an important factor in inducing hayfever.

W  SAPSUCKERS have been busy feeding on trees again this year.  These migratory birds come through each spring and peck rows of ¼” holes in trees, especially pines, birch and a few other species.  The birds then feed on the sap that oozes out.  Generally, by the time we see the holes, the birds are probably no longer in the area.  In most cases, the damage is not serious and the trees will be OK, but it may be severe enough where it can cause some dieback in the trees.  There is no treatment for the injury, the tree will have to take care of the wounds itself.  Some people have put hardware cloth around the trunks of trees that are attacked each year to try to discourge the birds.  Others have treated the trunk or limb with Tanglefoot below where the most recent damage has occurred.  The idea behind this treatment is to give the birds sticky feet when they try to sit on the tree to cause more damage.

W  Shearing pines is just around the corner.  When the needles on the candles are about one-half the size of the older needles is usually the optimum time for shearing.   At that time the candle can be broken or cut anywhere from one-half to a third its length.  Shearing at the proper time will create a smaller, denser pine – an excellent means of maintaining low mugo pines.