Pest Alert

June 5, 2000

Welcome to another issue of Pest Alert.  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 has shown up with recent rainy weather creating the proper environment for the disease to progress.  Since most trees had been leafed out for some time, less defoliation is likely to occur then when infection happens earlier in the season.  Treatment is rarely necessary or suggested.

W  Ash rust is showing up almost everywhere in eastern South Dakota but probably the worst cases are in the Huron to Aberdeen area.  The symptoms of ash rust are distorted leaves with bent  petioles.  The leaves are also covered with bright orange spots.  Heavily infected leaves will soon be falling.  This fungus disease is similar to cedar-apple rust in that it also requires an alternate host.  The fungus overwinters on marsh or cord grass then spreads in the spring to the ash trees.  At this time controls are ineffective.  Most trees will refoliate by the end of June so damage is often minimal.

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.

F  PEACH LEAF CURL is now active.  While this is probably a rarely seen disease, with the scarcity of peaches in SD, some home gardeners may be seeing this leaf distorting disease.  It is caused by a fungus that overwinters in bark crevices on the tree.  Typical symptoms include the curling and reddening of leaves and petioles with the underside of affected leaves having a whitish appearance.  No treatment is possible now, mark your calendars for next spring to apply lime-sulfur just prior to bud break.  Control is often very good with this treatment.

W  WESTERN GALL RUST is showing up in the Black Hills and has been seen in the Aberdeen area.  This rust disease causes orange-colored galls to develop on branches, often near the end of the branch where the needles begin.  When the galls are producing spores you will see them on your hands if you rub the gall.  The most commonly attacked trees are Ponderosa Pine and occasionally Mugo but other species may be affected.  Trees vary greatly in their susceptibility to the disease with some trees getting many galls while other trees nearby have none.  There is no good treatment for the disease except to prune out the infected branches.  However, an infected branch will probably  develop normal growth and may only occasionally be killed by the disease.  Best treatment is to live with the disease or completely remove the tree.

INSECT CONTROL

W  APHIDS continue to be a problem 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 flower gall mites are feeding on the male flowers of green ash trees.  The feeding from these mites results in the male flower clusters becoming branched and turning black.  Heavily infested trees look like they are covered with hundreds of tiny black balls.  While it may be unsightly, the galls do not harm the health of the tree so no control is recommended.

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 have begun emerging throughout South Dakota.  The dark coppery, torpedo-shaped adults are exiting out of the tree from D-shaped emergent holes and 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 they 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) now 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.

V  CUCUMBER BEETLES have begun feeding on  cucurbit seedlings.  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  Lecanium scales crawlers have become active in parts of South Dakota.  The crawlers begin moving along the twigs and branches of elms, ash and maples (among other hardwood species) about the time lindens begin to bloom.  The crawlers are visible with a hand lens but are difficult, if not impossible, to see with the unaided eye.  The crawlers will find a place to feed, develop a hard scale and become sessile.  Infested trees often produce a “sticky sap” this is the honeydew that is excreted by this scale as well as aphids and some other scale insects.  Heavily infested trees may have the leaves turn yellow and wilt; however, this is rare.  Generally control is not recommended, but if the tree does require treatment, Merit (imidaclorid) as a trunk or soil injection or superior oil are the best treatments.  These do not harm the natural enemies of the scale and these insects provide most of the control.

W  Longhorn beetle larvae are causing some concern in the eastern half of the state.  The large, often more than an inch, grub-like larvae can be found beneath the bark of dead trees.  Homeowners are finding these larvae under the bark and assuming that the insects are reason for the tree’s death.  Generally this is not the case as these insects generally infest dead and dying trees.  No control is necessary.

W  Tent caterpillars are causing big headaches for homeowners in the northeastern and central part of the state.  The Big Stone Lake region is seeing a significant increase in tent caterpillar populations from last year.  This is part of the typical cycle of the tent caterpillar, however.  The populations were high in these same areas about five years ago, then declined and now once again are on an increase.  The larvae, pale blue with white key-hole shaped markings on their back, are anywhere from ¼ to 1 inch in length at this time.  Mature larvae reach a length of about 1-1/4 inches.  Many of the larvae have left their webbed nests and are now moving to nearby trees.  These migrations can be surprisingly large involving hundreds of caterpillars hence the other common name – armyworms. If the larvae are still small, ¼ inch long, the best treatment is Bacillus thuringiensis also know as B.t.  If the larvae are larger B.t. may not be as effective and Orthene (acephate), or Sevin (carbaryl) may be a better choice.  If the larvae are more than 1 inch long, treatments are probably not warranted as the insect has completed most of its feeding and the damage is done.  Treatments are usually not necessary in any case as most trees can tolerate the early season defoliation.  These trees will begin leafing out again in the next couple of weeks.  

OTHER NOTES

W  It is now the time to shear pines in many areas of the state. When the needles on the candles – the new growth - are about one-half the size of the older needles, this 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.  Shearing spruce and fir, however, is best delayed until later in the season.