Pest Alert

July 28, 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

V  BACTERIAL WILT.   Typically infected cucumber, melon or squash plants or individual vines suddenly wilt during hot, dry weather and but may partially recover during the night.  Infected plants may be diagnosed by cutting a stem, near the base of the plant with a very sharp knife or razor blade.  Squeeze the cut ends and stick them back together.  Slowly pull the stems apart again and watch for the presence of tiny, sticky strands that will adhere between the two cut surfaces.  If these tiny strands are present, diagnosis for this bacterial disease is almost certain.

                No treatment is possible for infected plants.  The bacteria overwinter in cucumber beetles, which then spread the disease to young cucumber plants the next season when they feed.   Since cucumber beetle feeding was heavy this spring, lots of bacterial wilt is expected.  Early season control of the beetles should reduce the spread of the disease in the garden.  Remove Bacterial Wilt infected plants from the garden to reduce the chances of further spread of the disease.

V SQUASH MOSAIC and related mosaic diseases have been seen on squash and other cucurbits.  These virus diseases cause a mosaic pattern of green, lite green and yellow streaks to develop in the leaves initially.  A bit later in the season, the fruit will become distorted and discolored; typically exhibiting raised spots that may be a very prominent yellow color.   Infected fruit may also have an off-flavor. The disease can be spread by insects like cucumber beetles and aphids and also by physical contact or mechanical injury among the plants.  Once a plant is infected, there is no treatment for the disease other than to remove the plant from the garden to lessen the chances of spread to other healthy plants.

W  Dutch elm disease is showing a very large increase in the last week or two.  Several cities are now reporting a four- to eight-fold increase in the incidence of the disease.  Generally when we have a wet May the problem increases for the year.  According to Dutch elm disease researchers the wetter years result in the creation of a vascular system that is more susceptible to the development of the disease.   We have had several calls from people wanting to use ELMguard to save their trees, particularly those already marked by the city as having the disease.  ELMguard is not a fungicide but a product that is designed to increase the tree’s ability to fight infection (according to the ELMguard company).  It may be an excellent product and we hope to have more information on it later in the year.  However the company does not recommend treatments with ELMguard at this time of year or into fall, thus homeowners should not be expect this to save their infected trees.  If the city has marked a tree for Dutch elm disease the tree should be removed as soon as possible to reduce the chance of infecting nearby elm.

V  PURPLE BLOTCH OF ONION: Gardeners with a love of onions have seen a common disease developing over the past week or two. Purple blotch, a disease caused by an Alternaria fungus, has been seen causing dieback of the onion tops. Purple blotch can also affect garlic and leeks. The disease is expressed as dark spots on the leaves that may show a target pattern. Sometimes, if the day and night temperatures are not far apart, the ring pattern may not be as obvious.  As the spots grow larger, they develop a brownish to purple color. The leaves will yellow above the spots and as the spots girdle the leaves, the leaf will die back above that point. Purple blotch is a common disease of onions in South Dakota during periods of high temperatures and very high humidity. In years with those conditions it is probably even more common than Botrytis diseases.   Purple blotch will be more severe as the season progresses and leaves age. Sweet Spanish onions are among the most susceptible onion varieties. Fungicide applications with chlorothalonil can reduce the spread of purple blotch.

INSECT CONTROL

W  Aphids are still showing up everywhere and are found on almost any plant, though ash, elms and walnuts generate most of the calls.  Generally the concern is about a “weeping tree” or one that is dripping sap.  The culprit is not tree sap but honeydew, a byproduct of the aphids feeding on the tree leaves.  The honeydew is sticky and often dark colored due to a fungus – called sooty mold – that lives on the honeydew.  The feeding damage from the aphid is usually minimal and not necessary to control.  We have received reports of tree companies leaving flyers at homes stating that their trees have aphids and need to be sprayed.  In one case the notice said the birch had aphids that needed treatment but the tree was an aspen, not a birch, and there were no aphids!  While treatments are justified in some instances – excessive honeydew on the patio, perhaps, or a young tree that is heavily infested – generally no control is recommended.

F APPLE MAGGOTS should be emerging soon and begin laying their eggs on apple fruit.  The “railroad worm”, as it is called, is the larvae of a small insect that resembles a house-fly but has dark markings on the wings.  The adult emerges from the soil and goes off in search of apples.  The adult lays an egg just under the skin of the apple.  The egg soon hatches into the maggot that feeds inside the apple.  Infested apples will typically have a small black speck on the skin of the apple.  The apple may also be “dimpled” where the egg has been laid.  Small brown streaks will be seen  in the flesh of the apple where the maggot has fed.  Infested fruit, while unattractive, is still edible but has a much shorter storage life.  Sticky red ball traps may be used to monitor for the emergence of the adults.  Begin treatment when five adults have been captured on a sticky ball. Apply protective sprays of malathion, diazinon, Imidan or use a combination orchard spray to reduce infestations.  Some people have reported good results by simply using 6 to 8 sticky traps in a few trees in a yard to reduce damage to tolerable levels without using insecticides.

W  Leafcutter bees are making some very interesting patterns on lilac and rose leaves throughout the state.  The margins of these leaves look at if someone took a cookie-cutter to them.  The damage is due an insect that  looks like a very small bumblebee, that cuts circular pieces of foliage and uses these discs as a liner for the egg cells.   Actual damage to the plant is minimal as this is more a curiosity than a problem.

W  Mountain pine beetles are beginning to emerge in the Black Hills.  Traps started collected adults in early July, however, there has not been a big increase in activity since then – the peak is yet to come.  If you have individual trees you need to protect it is probably not too late if controls, Sevin XLR (carbaryl) or Astro (permethrin), are 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 still 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 STRIPED AND BLACK BLISTER BEETLES have been seen feeding on carrots, potatoes and other garden plants.  These tan and black striped beetles are about 3/4" long and about 1/4" wide.  They look like they have a very pronounced "neck" right behind the head.  They can feed in fairly large groups, quickly defoliating plants.  Care should be used around these pests since they can excrete a substance called cantharidin that can raise blisters on the skin.  Most garden dust formulations should help control these insects, also the liquid formulations of carbaryl (Sevin), malathion and diazinon should work as well.

 OTHER NOTES

V POOR FRUIT SET is a common concern among many gardeners on a number of vegetables.  Fruit set is a rather complicated process, dependant on many factors.  Weather, specifically temperature and moisture conditions being of primary importance.  If conditions are not right, fruit set will not occur.  Plant development and nutrition levels in the soil are also important factors.  Cucurbits are also affected by these same factors with the added dimension of having separate male and female flowers.  Generally, the first flowers to be produced on cucurbits are male, with more female flowers developing as the season progresses.  High temperatures will also greatly shorten the viable lifetime of pollen, decreasing the chances of ovules in the fruit from being successfully fertilized.  The general recommendation for all garden plants is to keep water stress at a minimum, do not over-fertilize and be patient.  A healthy plant will eventually set fruit.