Laurene Hall

Leaf Spots

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Source(s): Laurene Hall

Leaf spot is a common descriptive term applied to a number of diseases affecting the foliage of ornamentals and shade trees. The majority of the leaf spots are caused by a variety of fungal pathogens, but some are caused by bacteria.

Cercospora arachidicola- Early Leaf Spot (fungus)

Leaf diseases, including rusts and various types of leaf spots are abundant at this time of year. The good news is these foliage diseases usually reduce only the aesthetic value of the affected tree. Occasionally, however, a severe outbreak may cause early leaf drop and dieback of tree parts. With repeated infection, trees become more susceptible to attack by other diseases, insects, and winter injury.
Correct identification is important, because leaf spot diseases can be easily confused with non-disease problems such as leaf scorch or other environmental abnormalities. If in doubt, bring a sample for identification to your County Cooperative Extension Service.

IDENTIFICATION

The primary symptom of a leaf spot disease is spots on foliage. The spots will vary in size and color depending on the plant affected the specific organism involved,and the stage of development. Spots can vary in color from red, purplish-brown, tan, or black. Concentric rings or a dark margin around the spot may be present. Fungal bodies may appear as black dots in the center of the spots. Over time the spots may combine to enlarge to form blotches. Leaves may yellow and drop prematurely.
Leaf rust is another common problem associated with fungal leaf diseases. It is characterized by yellow spots on the upper leaf surface. Close examination reveals small yellow-orange bumps filled with powdery spores on the leaves. As with leaf spot, rust infestations will become apparent in mid to late August.

APPEARANCE

Leptosphaerulina crassiaca- Pepper Spot (bacteria)

Active in late spring to late autumn. The fungal and bacterial agents that cause Leaf diseases often exist naturally in the air and in our soils. The organisms that cause leaf spots survive in fallen infected leaves and twigs. Some may remain in dead twigs on the tree. During wet weather, spores are released which may splash or be windblown onto newly emerging tender leaves where they germinate in the moisture and infect the leaf. Overhead watering late in the day or during the night, heavy dews and close spacing of plants prolong wetting of the leaf surface and provide more opportunities for fungal or bacterial infections.

HOSTS

All species of trees, shrubs, and other ornamental plants can potentially develop leaf spots, but some species are more susceptible than others. It is the presence of the correct factors that will decide whether or not a plant will develop a Leaf disease from causal agents.
Three critical factors or conditions must exist for disease to occur — a SUSCEPTIBLE HOST, a PATHOGEN, and the right combination of POOR ENVIRONMENTAL CONDITIONS. The relationship of these factors is called the DISEASE TRIANGLE.
If only a part of the triangle exists, disease will not occur. Understanding the disease triangle helps us understand why most plants are not affected by the many thousands of diseases that exist.
In some cases, it is important to only acquire specific varieties of plants that exhibit a resistance to diseases that are particularly fatal.

INTEGRATED PEST MANAGEMENT (IPM) CONTROL

Live with the disease. Leaf spots are largely an aesthetic problem as few leaf spots seriously damage the host. Here are some steps to follow that will help control the disease after the infection has started.

  1. Remove infected leaves and dead twigs. The fungi responsible for these diseases survive the winter in leaf debris on the ground. Wet spring weather stimulates spore production. The spores are blown and splashed from the ground to developing leaves. Foliage diseases are cyclic: bad years coincide with long-term moist weather. Raking up and disposing of infected leaves as they drop and pruning out dead twigs can help control the disease by removing spores that can re-infect the new leaves. This will not cure the problem but it can help minimize infections.
  2. Keep foliage dry. Avoid overhead watering. Use soaker hoses or water early in the day so the foliage can dry before night. Watering can also spread the disease by splashing. Prune plants and space plants to allow for good air circulation that promotes rapid drying of foliage.
  3. Keep plants healthy. Since most plants can tolerate some defoliation, keep them in good health so they can rebound quickly. Avoid over fertilization as it promotes a flush of young leaves that are more susceptible to attack by insects and disease.
  4. Replace the plant. For plants that chronically are plagued by leaf spots, gardeners find it more convenient to replace a plant with a different species or a variety that is more resistant or tolerant of disease.

CHEMICAL CONTROLS

Use fungicides wisely. Chemical control is usually not necessary and often unsuccessful if the fungicide is not applied properly. It is usually impractical and not economically feasible for homeowners to spray fungicides on trees. This is because many trees that are affected by leaf diseases are well-established and too large to spray. Additionally, mature trees may naturally develop leaf diseases more frequently as a result of the stress of old age; consequently, if healthy enough, the tree may recover from a leaf disease on its own and without a homeowner’s intervention. In rare cases of severe infection where the size and value of plants make it practical, applications of fungicides may be helpful.

Generally fungicidal control is warranted if:

  1. repeated defoliations occur in one year or subsequent years;
  2. the plant is under stress or in serious decline and showing no signs of recovery with improved environmental conditions;
  3. or there is serious danger of infection spreading rampantly to other uninfected plants.

Sprays will not cure the infection but protect leaves from becoming infected. To be effective, fungicidal sprays must begin at bud break before symptoms are noted and be continued at intervals specified by the label (usually 10-14 days) through the period of spring rains. Spraying after the infection is present will provide little benefit. Recommendations will vary with the disease and fungicide used. It is always good practice to have the disease identified before purchasing a control product. If you think your plant is suffering from a Leaf disease, bring a sample of the leaves on a branch with both healthy and diseased tissue to the Extension Office for proper diagnosis. A fungicide recommendation may be given if the problem cannot be corrected by cultural IPM practices. Always carefully follow label directions when applying chemicals and wear protective clothing if necessary.

Resource(s): Common Landscape Diseases In Georgia

Center Publication Number: 47

Needle Blight

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Source(s): Laurene Hall

There are several types of Needle Blight diseases that effect plants. Pestalotiopsis sp. is a type of needle blight fungal disease that is considered usually a minor disease. It attacks foliage that has been injured or weakened by unfavorable weather or growing conditions. Usually, the killed foliage is near the base of the plant and where foliage is relatively dense. The disease may kill the smaller twigs where the infected needles died.

needle

IDENTIFICATION

The Blight usually starts at the tip of the foliage and progresses towards the leaf base. The color of the foliage goes from green to yellowish, then to a dark brown that can look almost black. Twig tips turn tan to brown in color and have black, pimple-like fungal fruiting structures dotting their surface

APPEARANCE

Infections can occur anytime of the year. In the spring, when weather conditions are favorable, fungal populations can increase very rapidly and disease losses can be severe. Factors that favor disease include frost during early shoot development; cool, wet weather in spring or fall; pruning wounds; and heavy vegetative growth early or late in the season brought on by high nitrogen applications. Most importantly, this disease often affects trees that have suffered from stress (whether it is cultural, mechanical or environmental).

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HOSTS

Various species of trees, shrubs, and other ornamental plants can potentially develop the disease. However, some plants are more susceptible than others. Particularly, needled evergreens, conifers, and some varieties of juniper are most susceptible. The presence of the correct environmental factors will decide whether or not a plant will develop this disease from causal agents. Additionally, wind-driven rain, insects and contaminated pruning tools all spread the fungus from plant-to-plant

INTEGRATED PEST MANAGEMENT(IPM) CONTROL

Control is to prune out dead foliage and twigs as soon as possible. Reduce winter injury by minimizing dehydration. Do not allow snow to accumulate or remain up and around the base of the plant for any length of time – especially during melting. Avoid over crowding the plants to allow sufficient sunlight and air circulation. Keep plants healthy by maintaining good cultural practices (i.e. proper watering, fertilization and minimal mechanical damage) and, when possible, protecting plants from other environmental stresses such as drought.

CHEMICAL CONTROLS

Use fungicides wisely. Chemical control is usually not necessary and often unsuccessful if the fungicide is not applied properly. It can also be impractical and not economically feasible for homeowners to spray fungicides on trees. This is because many trees that are affected by Blight are well established and too large to spray. Additionally, mature trees may naturally develop the disease more frequently as a result of the stress of old age. Consequently, with the proper pruning and removal of infected material the tree may recover from the blight on its own and without the use of a fungicide.
If you think your plant is suffering from blight, bring a sample of the leaves on a branch with both healthy and diseased tissue to the Extension office for proper diagnosis. A fungicide recommendation may be given if the problem cannot be corrected by cultural IPM practices. Always carefully follow label directions when applying chemicals and wear protective clothing if necessary.

Resource(s): Common Landscape Diseases In Georgia

Center Publication Number: 48

Dollar Spot

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Source(s): Laurene Hall

Dollar Spot makes circular areas only a few inches in diameter. Spots may run together causing large, irregular patterns. Blades have straw-colored lesions along one edge that spread across the leaf blade until tips dieback. Leading edge of dieback is reddish brown. White mycelium (the vegetative part of a fungus that looks like a mass of branching strings) may be associated with patches when turf is wet.

3dolspt1 DollarSpot3

Appearance

Active in late spring to late autumn. Will attack lush turf fertilized with excessive nitrogen (<1/2 lb. N/1000 sq. ft./month). Disease is severe in soils that are low in phosphorus and potash; when night temperatures are 50 to 60° F (10 – 16° C) and when more than 10 hours of foliar wetness persists per day for several days. Disease is also severe when the soil drains poorly and turf has previously suffered from other environmental stresses

Hosts

Bentgrass, Bermuda, Centipede, Fescue, Rye, St. Augustine, Zoysia

IPM Control

Maintain adequate nitrogen fertility. 1/2 lb. to 1 lb. of N/1000 sq. ft. every 2 – 4 weeks will reduce severity of disease. Maintain moderate to high levels of soil potassium as determined by soil tests. Decrease shade and increase air circulation to enhance drying of turf. Avoid irrigation in late afternoon and in evening prior to midnight. Water only when required to depth of 5 – 7 inches in early morning. Maintain thatch at ½ in. thickness or less. Raise mowing height.

Chemical Controls

If you think you have Dollar Spot in your lawn, bring a sample of grass (about 4 in. square) with both living and dead tissue to the Extension office for proper diagnosis. A fungicide recommendation may be required if the problem cannot be corrected by cultural IPM practices. Always carefully follow label directions when applying chemicals and wear protective clothing if necessary

Resource(s):

Center Publication Number: 46

Azalea Leaf Gall

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Source(s): Laurene Hall

Azalea leaf gall is a very common and widespread fungus disease that occurs in early spring on new azalea foliage. The leaves become thickened, curled, fleshy and pale green to white in color. Fortunately, this disease is more alarming than damaging.

Dsc-0028-06AzaleaLeafGall

Description

Caused by a fungus, Exobasidium vaccinii, which is dormant in the developing buds from one year to the next. When bud growth begins in the spring, the pathogen renews activity also, and one or more of the leaves on the shoot may develop symptoms. A spore-bearing hymenium is eventually produced which completes its development on exposed leaf surfaces. Spores are blown about by air currents, some of them lodge on the plant and finally invade leaf buds. The actual damage to plants is not nearly so important as it appears to be. However, if disease is severe, the vigor of the plant can be affected due to the loss of young leaves.

Detection

First noticed soon after leaf buds open in the spring. All or only part of the individual leaf may be affected. Part of the leaf becomes distorted with a pale green to whitish, bladder-like thickening. When young, the thickened, fleshy-like leaf is covered with a white growth. As the galls age they turn brown, dry up and fall to the ground. Occasionally, a black coating may develop on the surface of the gall, particularly during rainy weather, which results from secondary invasion of the galls by the fungus Pestalotia.

Control

  1. Handpick or prune out and destroy (burn) galls.
  2. Spray with recommended fungicides, such as Daconil 2787 or Mancozeb.
  3. Serious outbreak in large plantings: spray in early spring when leaf buds just begin to open and at two-week intervals (if spring is relatively dry) through early summer (mid-June) with Bordeaux mixture which may reduce incidence of disease in the following season. Timing of sprays is critical because the spores over-winter in the bark and bud scales.

Resource(s): Common Landscape Diseases In Georgia

Center Publication Number: 44