Pest Alert

July 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 Ash rust is still showing up almost everywhere in eastern South Dakota.  The symptoms of ash rust are distorted leaves with bent petioles.  The leaves are also covered with bright orange spots. The heavily infected leaves have already started to fall.  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 not necessary as the trees can generally tolerate this injury and will recover by next spring.  

F,W  FIRE BLIGHT infections are now quite evident on trees and should be treated by pruning out any signs of the infection.  The most susceptible plants include apple, pear, crab apple, mountain ash and cotoneaster.  Dip the pruning saw or shears in a 10% bleach solution between cuts and thoroughly rinse and dry tools when you are finished.  Blossom or spur infections can be numerous making pruning difficult.  In severe cases, especially where other susceptible plants are in the same yard, consider completely removing the most severely infected plants.

V  LATE BLIGHT continues to be a problem in gardens.  Large blotches of dead leaf tissue are characteristic of late blight. The dead spots cross leaf veins, unlike any other disease of potatoes and tomatoes. Often the dead areas will be bordered by a yellow halo. On the underside of the leaf, a white, cobwebby growth may be present during wet weather. Dark, streaky lesions may also be present on the stems of either tomato or potato.

The disease can be controlled in the home garden by picking off diseased leaves and using fungicides such as chlorothalonil or mancozeb that are approved for use on garden vegetables. Severely infected plants should be removed from the garden. Many brands of fungicide with these active ingredients are available from garden centers and discount stores. Applications as often as every 7-10 days may be needed during periods of cool, wet weather.  Similar control strategies will be effective for Early Blight and Septoria Leaf Spot.  See the last issue of Pest Alert for more details.

INSECT CONTROL

W  Aphids are showing up everywhere but are probably most noticeable on ash trees.  Numerous reports from Spearfish to Elkton are coming from homeowners and farmers wondering why the leaves near the branch tips are curled.  While many suspect herbicides, if you open one of the curled leaves you’ll generally find a colony of aphids.  The feeding injury of these aphids includes leaf curl as well as yellowing.  Generally the damage is minimal, shoot growth is already completed and there remain sufficient uninjured leaves to support the energy needs of the tree.  The tree does not look very nice, however, so some people do spray the tree.  If treatments are applied consider the fact that the curled leaves protect the aphids – contact insecticides will not reach them.  Instead use an insecticide with some systemic action such as Orthene (acephate).

W  Ashgray blister beetle adults are feeding on peashrubs and honeylocusts from Mitchell to Spearfish.  The ½ inch long adult beetles are gray to black and have elongated bodies.  The adults feed in colonies and can defoliate a small honeylocust in less than a day.  They are slow movers, however, so you can easily pick them from the tree or shrub BUT DON”T!  The beetles can spray a chemical substance called cantharidin, which can produce large blisters on exposed skin.  The beetles are also toxic to livestock if accidentally ingested.  Horses have been killed by eating alfalfa hay containing beetles.   Recommended treatment for ornamental plants is Sevin (carbaryl) applied when the adults are first seen.  The larvae of the blister beetles are beneficial as they feed on grasshopper and other insect eggs.  

V  CUCUMBER BEETLES are actively feeding on young cucumbers, pumpkin, squash and melon plants.  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.   Treatment with Carbaryl (Sevin), Esfenvalerate and rotenone containing products should be effective.

F  GRAPE PHYLLOXORA will be showing up on grapes in many areas.  This tiny aphid-like insect lays it eggs on grape leaves causing the leaf to develop a gall on the leaf underside.  The young larva develops inside the gall, eventually laying eggs.  Once the eggs hatch, the larvae crawl out and start new galls on other leaves.  While the galls are unsightly and can cause leaf malformation, the pest is usually not very damaging to the plant.  No treatment is really feasible.  Another more damaging form of phylloxora attacks the roots and can stunt and kill grape plants but this form of the pest is usually not a problem in modern grape varieties.

V  COLORADO POTATO BEETLES continue to plague home gardeners.  Carbaryl (Sevin), Esfenvalerate and rotenone can be effective but there is a lot of potential resistance problems.  Bacillus thuringiensis var. tenebrionis (M_Trak) is an option.  There are no known resistance problems to it, but Bacillus is only effective against very young larvae.  Keep potatoes well watered to decrease susceptibility to defoliation.

W  Mountain pine beetles are beginning to emerge in the Black Hills.  Traps started collected adults on Monday (July 3).  If you have individual trees you need to protect Sevin XLR (carbaryl) or Astro (permethrin) should be applied now.  Generally pines in towns are too far away from infestations to need protection. However, if the trees are within ¼ to ½ mile of a current infestation control may be valuable.  Homeowners will need to contact a commercial applicator since the lower 30 to 50 feet of the tree will need to be treated.

W  Poplar gall aphids are causing premature defoliation of cottonwoods across the state.  While there are a number of insects responsible for early leaf fall on cottonwoods these are probably the most noticeable due to the large gall at the base of the fallen leaf or leaf stalk.  If the galls are broken open you can still find the small aphids responsible.  These will soon be leaving the galls so don’t be surprised if the galls turn up empty.  No chemical controls are currently registered for this problem.

V  SQUASH VINE BORERS are becoming active across the state.  These attractive bluish moths, which look like a wasp, deposit eggs near the base of squash, pumpkin, and occasionally melon vines.  Newly hatched larvae immediately burrow into the stem where they continue to feed until they reach approximately one inch in length.  This feeding eventually kills the vines, although it appears to happen quickly because the vines suddenly wilt.  Vines must be protected from late June through mid August from egg laying adults and newly hatched larvae with an insecticide application to the base of plants.  Dust formulations are probably best for this insect, including Sevin (carbaryl), rotenone, and methoxychlor, which is available as Ortho Vegetable Garden Dust.

 OTHER NOTES

V  BLOSSOM-END ROT will soon arrive in many home gardens.  This physiological disease is caused by a calcium deficiency in the fruit.  The plant takes up less calcium than it should during times of rapid growth.  This often happens during wet weather following a dry period.  Since there is not enough calcium in the plant, the end of the fruit doesn’t get enough to develop properly.  Typically, determinant tomato varieties are more severely affected since much of the fruit develops at one time.  The best way to treat the problem is to discourage growth spurts from fluctuating soil moisture levels.  Use a mulch to keep soil moister during dry periods, water during dry periods and try using semi-determinant and indeterminant tomato varieties.  Applications of calcium are generally not effective since our soils generally have adequate levels of calcium.

V  ROTTING SQUASH fruit generally means that the female flower was not adequately pollinated to allow for the development of the fruit.  Without pollination, the fruit may survive for a couple days but then begin to rot from the blossom end.  This is NOT like blossom-end-rot which is found in tomatoes and peppers.  Extremely heavy, dense vine growth, wet conditions, odd weather patterns, excessive use of insecticides or just a lack of pollinating insects can increase the likelihood of this problem occurring. Generally the problem will disappear on its own. Also, warmer temperatures will promote the production of more female flowers to allow for more potential fruit than during cooler weather.  Incidentally, bumble bees are much more effective pollinating insects for cucurbits than honey bees.  Cucurbits, in general, can also suffer from blossom-end rot, much like tomatoes and peppers.  Treatment is the same - maintain even moisture conditions in the garden to facilitate even uptake of water and nutrients, including calcium.