Welcome to Pest Alert. Categories for topics are as follows, H - Herbaceous plants and flowers, W - Woody Plants, V - Vegetables, F - Fruit and L - Lawns.
DISEASES
H,V ASTER YELLOWS, HOPPERBURN, AND LEAFHOPPERS: This is the time each year when we begin to notice that beans in the garden are showing a puckering of the leaf and potatoes may be showing a burn on the edges of the leaves. A V-shaped "burned" area may also develop at the tip of potato leaves. Some of this injury may be due to feeding injury from the potato leafhopper. Potato leafhoppers entered the area about two to three weeks ago, traveling on upper level winds from their overwintering grounds in southern Louisiana. Leafhoppers are very active, wedge-shaped insects, about 1/8 inch long. Insecticides used for aphid control are generally effective against leafhoppers. If leafhoppers have moved onto other feeding sites, insecticidal controls are unnecessary.
Another similar leafhopper may transmit a disease called Aster yellows. Aster yellows is vectored by the Aster leafhopper, sometimes also called the six-spotted leafhopper. Aster leafhoppers overwinter in Louisiana, Arkansas, and eastern Oklahoma and are blown northward on the same winds that carry the potato leafhoppers to the area. The direction of the wind currents in any given year will determine if either leafhopper arrives in numbers adequate to cause problems. Aster yellows may occur on many hosts in the flower and vegetable garden. Potatoes develop diseases called haywire and purple top wilt. Carrots may develop aster yellows or hairy root. Many flower species including petunias, marigolds, zinnia, coneflowers, and many other composites may develop aster yellows. Aster yellows may be expressed as a general yellowing of the leaves and in some cases the flowers may become reduced in size, greenish in color and somewhat leaf-like.
F BROWN ROT OF STONE FRUITS: Brown rot fruit mummies are beginning to appear on plum and apricot trees in the state. Brown rot is one of the big three diseases of stone fruits along with black knot and plum pockets. Apricots are probably the most susceptible stone fruit species to brown rot. Brown rot may cause branch tip dieback in a wet spring. As such it is important to remove the fallen fruit under stone fruit trees and remove the shriveled, dried, fruit mummies from the trees in the fall. These old fruits will serve as overwintering sites for the fungus to lead to disease next spring. The disease may also be managed with fungicide applications during the bloom stage. However, at this point in the season, sanitation is the most important control measure.
W CERCOSPORA BLIGHT has been observed on eastern redcedars and Rocky Mountain junipers throughout the state. While infections usually occur in the spring, the symptoms for the new infections do not begin to appear until about mid-July or August. The typical pattern to cercospora blight is bronzing and dying of the interior branches. The two other common juniper diseases, Kabatina and Phomopsis tip blight, generally have the browning start at the branch tips and spread inward. Most of the spores for cercospora are released during the moist spring weather. Thus the best time for control is an application of Mancozeb in late May followed by a second application in early July. Spores are not released during the hot, dry weather of mid to late summer so control now will be ineffective.
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. 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 Bordeaux mixture or chlorothalonil in the spring before bud break to protect the older needles and a second time in the mid summer to protect the new needles. Most infections occur during wet periods in mid summer. 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 LATE BLIGHT FRUIT ROT ON TOMATO: Late blight continues to be a problem on tomatoes, recently causing infections on tomato fruits. The infected fruits develop a dry, brownish granular appearance on the skin. When cut open, the fruit are discolored internally. It is a dry rot initially, but as the disease spreads inward the fruit will begin to break down. It is critical to destroy these infected fruits and other infected plant parts. Be careful not to leave the infected residue in the garden. The fungus will continue to sporulate and cause new infections.
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 among the most susceptible onion varieties. Fungicide applications with chlorothalonil can reduce the spread of purple blotch.
W SIROCOCCUS SHOOT BLIGHT symptoms are showing up on blue spruce in eastern South Dakota. This disease is not very common in our state, but with the continuing pattern of wet, cool springs there has been an increase in infection over the past five years. The disease results in tip dieback. The needles are shed along the current year's growth and the twig may curl and become hook-shaped. These symptoms may be confused with with frost, herbicide or spruce needleminer. The difference is that frost and herbicide injury usually create a more uniform pattern to injury while with sirococcus the pattern is more random. Usually tips here and there throughout the canopy are injured. Needleminer damage can be separated from sircoccus by the small webs that are associated with the lost needles. Sircoccus can also be identified by the small black fruiting bodies that form on the buds and dead twigs. Control recommendations for this disease are limited to two applications of Chlorothanlonil, one when the new shoots are about ½ inch long and the second about four weeks later.
W TAR SPOT OF MAPLE is appearing on maples, particularly silver maples, in many parts of South Dakota. The symptoms are greenish-yellow spots that develop on the upper leaf surface during early summer. These spots become tar-like by mid-summer hence the name 'tar spot." While the disease may result in some premature defoliation, the problem is not likely to result in significant injury to the tree. No control is recommended.
INSECTS
F APPLE MAGGOTS should be emerging and begin laying their eggs on apple fruit. The "railroad worm," as it is called, is the larva 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. 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. Or, apply protective sprays of malathion, diazinon, Imidan or use a combination orchard spray to reduce infestations.
V BEAN LEAF BEETLES are coming around again for the second time this year. These small yellow colored beetles have black spots and two lines along the edge of the back. They can chew numerous small 1/4" holes in leaves or, if feeding in large numbers, eat most of the plant in a few days. Treatment with sevin or rotenone should be effective. Do a thorough harvest before treatment to allow for the harvest interval stated on the label.
F CODLING MOTH damage to apples is already showing up this year. The most common sign of damage is a small bit of, what appears to be "sawdust" at the blossom end of the fruit but this may be seen on other areas of the fruit too. Fruit malformation is also common. The caterpillar typically feeds in the core of the apple but may be found tunneling in other parts of the fruit as well. This is the typical "worm" that people will find in their fruit. The caterpillar is usually about 1/2" long and tan in color. Once the caterpillar is inside the fruit, no control is possible. Insecticides like Imidan, malathion, sevin, or diazinon or the use of a combination orchard spray should reduce damage next year if the spray is present on the fruit before the larvae can bore inside.
W COOLEY SPRUCE GALLS have become evident on blue spruce trees. The pineapple or conelike, reddish galls can be found on the curled tips of blue spruce trees. The cooley spruce gall adelgid (aphid) feed inside these galls which can be found on blue spruce or Douglas-fir trees. While the galls will not kill the tree, they can be unsightly. Best control is to simply prune them out of the tree.
W ERINIUM GALL MITES are actively feeding on some silver and sugar maples. These very tiny mites feed in small colonies forming bright red patches on the undersides of leaves. Like the maple bladder gall mites, these cause virtually no damage to the tree so no treatment is recommended.
OTHER NOTES
V BLOSSOM END ROT REVISITED: Blossom end rot is not a disease as such. It is a disorder caused by calcium deficiency brought on by fluctuations in soil moisture, particularly moisture stress. Excessive amounts of nitrogen fertilizer (especially ammonium) and root pruning may also contribute to blossom end rot. Blossom end rot can be minimized by:
Keeping plants uniformly supplied with water -- avoid wide cycles of wet and dry. Use a plastic or organic mulch, as this helps provide a more uniform moisture supply. If rows are 5 feet apart, the plastic mulch should be 4 feet wide in clay soils and 3 feet wide in sandy soils. The edges of the plastic should be weighted down with soil to prevent the wind from getting under the plastic. This can be done by digging a shallow trench around the edge of the plastic, placing the edge in the trench, then covering the edge with soil. Small holes should be cut in the middle of the plastic to plant the tomato transplants. It is best to use clear plastic which warms the soil faster in spring, producing a better root system; it may even result in an earlier crop of tomatoes. When transplanting the tomatoes, be careful that the plastic does not touch the plants as this can result in heat injury to the stem. A space of at least 1/2 inch should separate the plastic from the plant stem. In a rainy growing season, the soil under plastic may remain too wet too long, setting the stage for root disease development.
Minimizing excessive nitrogen fertilization -- use a fertilizer high in phosphorus such as superphosphate and containing a nitrate form of nitrogen. A fertilizer such as 4-12-4 or 5-20-5 is good for tomatoes.
Planting tolerant tomato varieties -- Indeterminant varieties (those that produce fruit throughout the season rather than in a short flush) typically have less of a problem with blossom end rot because they have less water demand in a short period of time.
Protecting the roots from injury -- avoid cultivation too close to the plants.
Since this disorder is not caused by a pathogen, moving the your tomatoes to a different part of the garden each year will not affect the amount of blossom end rot in the garden. Moving the plants each year is a good practice to control wilt diseases, but it will not reduce blossom end rot.
L,W,V SLIME MOLDS have been appearing in urban ornamental plantings for the past few weeks. Most of these fungi look like patches of yellow or brownish goo that changes to a dusty mass of brown or gray. Some have been described as looking like a spilled chocolate malt. The slime molds we see most often in the landscape are usually in the genera Mucilago and Fuligo. These fungi live in moist locations, contributing to the decay of organic matter. Often, wood mulches provide a favorable habitat for the vegetative stage of the fungus. While the vegetative stage is growing, the slime mold is inconspicuous, growing below the surface. Once a year it will move to the surface to "fruit" or produce its reproductive structures. Just as an apple tree produces apple fruit with seeds and mushroom fungi produce mushrooms with spores, these fungi produce a spore producing structure filled with dusty gray to brown colored spores. These fungi are not harmful to plants or man, though they may be considered unsightly. They are usually only visible for up to about a week each year. Most of the time they're just trying to make a living by eating the things that few other organisms in this world are interested in!
W,F STORM DAMAGE. During the recent storms that have occurred across the region there have been many examples of 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,F SUCKER REMOVAL. While major pruning (removing more than 15 to 20 percent of the foliage) should be deferred until the dormant season, now is an excellent time to remove suckers. Suckers are the shoots that originate at the base of the tree. These shoots are from the rootstock and will not have the same characteristics as the cultivar. These shoots, if allowed to develop, may overtake the cultivar and the tree will revert to the characteristics of the rootstock. While it is impossible to prevent these suckers from continuing to develop, those removed during the summer come back with much less vigor.
V,H,L,W WATER PROPERLY when the plants need it. Generally most lawns, gardens and flowers need about 1" of water per week. If your area has not been receiving this amount, supplemental watering may be needed. The important point to remember is to water thoroughly when you do water. Try to saturate the root zone of what you are watering. This means applying at least 1/2" at one time, preferably more like 1" should be applied at one watering. Then allow the soil to dry before watering again. You can tell when it is time to water by sticking your finger into the soil to see if it feels moist in the top 2" of soil. If it is dry, it is probably time to water. Watering from below is generally better than overhead. This can be done using soaker hoses or a drip system. Overhead watering tends to waste water and can promote disease problems if the foliage is allowed to go into the night wet.
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Most recent revision Wednesday, July 29, 1998 by David F. Graper.