Identifying and Controlling Most Common Spring Plant Diseases in the Landscape

by Alfredo Martinez, University of Georgia,
Department of Plant Pathology-Griffin Campus

Rust on crabapple (Malus)

In the following paragraphs are described some of the most common spring diseases we have started to see and that you may encounter in spring:

1. Camellia Leaf Gall

The disease is caused by the fungus Exobasidium camelliae. Leaf gall causes little damage to the overall health of the shrubs or to camellia flowering, but may be unsightly in the landscape or garden.

The severity of the disease varies according to the weather conditions when leaf expansion begins in the spring. Cool, moist weather favors disease development. Frequently overhead sprinkler irrigation provides the moisture necessary for disease development. The fungus survives during the winter in leaf buds and infects the developing leaf tissue. Instead of developing normally, the new leaves become thickened and succulent and may be larger than normal. These young, diseased leaves have a pinkish-green color on the top leaf surface and a white color on the under surface. Spores are released and dispersed by air currents and splashing water. The diseased leaves dry up and turn brown to black in late spring.

Control:

• Removing and destroy/discard diseased leaves as they appear.

• Prune infected branches from the plant as soon as symptoms are observed, but before the lower leaf surfaces turn white and spores are liberated.

• Infected branches should not be left on the ground after pruning because spores can be generated and dispersed from the infected clippings.

• Fungicide applications are seldom necessary and provide only limited disease control. If necessary, fungicides need to be applied preventively as the leaf buds swell in the spring. Consult Texas Cooperative Extension for fungicide recommendations.

2. Cedar-Apple and Quince Rusts

Cedar-apple rust and quince rust affect two groups of vastly utilized landscape plants. The cedar-apple rust fungus overwinters in galls that may grow to several inches in diameter on eastern red cedar and several other junipers. In the spring, brightly-colored, gelatinous horns emerge from the galls during wet weather. These horns consist of masses of spores that are spread by wind to newly-emerging apple, crabapple, and hawthorn leaves and fruit. By mid-summer, rusty or orange-colored spots appear on infected leaves. In mid-to late-summer, spores produced in these spots are carried by the wind to cedar and juniper. On susceptible crabapple cultivars, rust causes premature defoliation, stunted growth.

The disease cycle of quince rust is similar to cedar-apple rust. The galls of quince rust on eastern red cedar and other junipers are small and spindle-shaped. Quince rust affects fruit, young stems, and petioles on Rosaceous hosts such as apple, crabapple, hawthorn, quince, mountain ash, and cotoneaster. Fruits are stunted and killed, and twigs and petioles become swollen and distorted, often resulting in death.

Control:

• On coniferous hosts, prune affected branches 6 to 8 inches below galls during dry weather with sterilized pruning tools.

• Use cultivars of crabapple and other Rosaceous plants that are resistant to rusts.

• On juniper, apply mancozeb, or mancozeb plus thiophanate-methyl according to label recommendations. On rosaceous hosts, apply mancozeb, triadimefon, or mancozeb plus thiophanate-methyl according to label recommendations.

3. Fireblight

Fireblight is a destructive, highly infectious and widespread disease caused by the bacterium Erwinia amylovora. The disease affects plants in the Rosaceae family: Malus (apple and crabapple), Cotoneaster (cotoneaster), Prunus (flowering almond, plum and cherry), Chaenomeles (flowering quince), Crataegus (hawthorn), Eriobotrya (loquat), Sorbus (mountain ash), Photinia (photinia), Pyracantha (pyracantha) Rosa (rose), Amelanchier (serviceberry), Spirea (spirea) among others.

The bacterium spends the winter in sunken cankers on infected branches. In spring the bacteria ooze out of the cankers and attract bees and other insects. The bacteria spread rapidly through the plant tissue in warm (65ºC or higher), humid weather. Insects, rain, wind and pruning tools can spread the disease.

Fire blight attacks blossoms, leaves, shoots, branches, fruits, and roots. Initially the disease often enters the tree through natural openings, especially flowers and wounds in the spring. Once established in the tree fire blight quickly invades through the current season's growth into older growth. Young twigs and branches die from the terminal end and appear burned or deep rust colored. Branches may be bent, resembling a shepherd's crook. Dead leaves and fruit remain on the branches. The bark at the base of blighted twigs become water soaked, then dark, sunken and dry; cracks may develop at the edge of sunken area.

Control:

• Prune out infected branches 8 inches below the signs of damage.

• Avoid pruning when the plants are wet. Dip pruning tools in 70% isopropyl alcohol (rubbing alcohol) or 10 % Chlorox solution (1 part bleach to 9 parts water solution) between each cut.

• Avoid heavy nitrogen fertilization, especially in summer, succulent growth is especially susceptible to fireblight infection.

• Avoid splashing water.

• Chemical control is not always effective and needs to be applied preventively. If needed, spray plants with a fungicide containing basic copper sulfate such as Kocide or an antibiotic such as Agrimycin to reduce infection. Applications of Agrimycin need to begin at the start of blooming and continue every 3-4 days during the bloom period. Application of Kocide should begin at bloom and continue every 7 days during bloom. Re-application following rain may be needed.

• Plant resistant varieties.

4) Dogwood (Cornus florida) Anthracnose

Dogwood anthracnose is caused by the fungus Discula. Tan-colored leaf spots with purple margins form on developing leaves and flower bracts. These spots grow together, forming large blotches on leaf blades and along leaf margins. Infected leaves eventually die. The fungus may continue to grow down into the petioles and branches, resulting in the death of twigs and branches. Brown, elliptical cankers may form at the base of dead branches. Drought, winter injury, and environmental stress predispose dogwood to anthracnose. Kousa dogwood (Cornus kousa) is resistant to this disease.

Control:

• Improve plant vigor, avoid moisture stress, avoid wounding, and prune affected branches 6 to 8 inches below diseased tissue during dry weather with sterilized pruning tools. Avoid planting dogwoods in shady or crowded areas.

• Chlorothalonil provides fair control of the leaf spot phase of this disease only. Apply the fungicide according to label.

5) Dollar Spot

The disease is caused by the fungus Sclerotinia homoeocarpa. Susceptible turfgrass include: all species of warm- and cool-season turfgrass. Tall fescues, Bentgrass, Zoysiagrass and Bermuda hybrids are particularly susceptible.

Dollar spot causes sunken, circular patches that measure up to few inches on turfgrass. The patches turn from brown to straw color and may eventually coalesce, forming irregularly shaped areas. Infected leaves may display small lesions that turn from yellow-green to straw color with a reddish-brown border. The lesions can extend the full width of the leaf. Multiple lesions may occur on a single leaf blade. Abundant white fungus growth may be seen in these areas during periods of severe disease development.

Dollar spot is favored by temperatures between 60°F to 85°F and continuous high humidity and low soil moisture. This disease is particularly favored by warm days, cool nights, and intense dews. It also infects areas with low levels of nitrogen and becomes more severe in dry soils. Dollar spot is more common during the spring and fall months.

Control:

Management practices helpful in controlling this disease include the addition of nitrogen and providing sufficient soil moisture.

• Use an adequate level of nitrogen, particularly in the spring and early summer.

• Mow grass at regular intervals.
• Irrigate turf early in the day to allow the foliage to dry as quickly as possible.

• Reduce thatch.

• Increase the air circulation.

• Irrigate turf deeply and as infrequently as possible to avoid drought stress.

• Remove dew from the turf early in the day. Fungicides can be used to help bring the disease under control once it is established. Consult current Extension treatment handbooks.

6. Brown Patch

Brown patch is caused by Rhizoctonia solani. Susceptible turfgrass include all species of warm- and cool-season turfgrasses including St. Augustine grass, Zoysiagrass, Bentgrass, Ryegrass, Centipede and Bermudagrass. It is one of the most common turfgrass diseases in the State.

The symptoms of brown patch can vary depending on the grass cultivar as well as climatic and atmospheric conditions and soil of the turfgrass management. This disease typically causes rings and/or patches of blighted turfgrass that measure 5 inches to more than 10 feet in diameter. It also causes leaf spots and "smoke rings" which are thin, brown borders around the diseased patches that appear most frequently in the early morning. After the leaves die in the blighted area, new leaves can emerge from the surviving crowns. On wide bladed species, leaf lesions develop with tan centers and dark brown to black margins.

The most favorable conditions for disease development usually occur from late April through October. Brown patch favors high relative humidity as well as temperatures of over 80°F during the day and over 60°F at night. This disease can be quite active on warm season grasses in the spring and fall. It also occurs in areas that experience more than 10 hours a day of foliar wetness for several consecutive days. Brown patch infestation is more severe when the turf is cut to a height less than the optimum for the turfgrass being grown. Heavy nitrogen applications increase the susceptibility of the grasses to fungus infection.

Control:

• Management plays the most important role in brown patch control.

• Use low amounts of nitrogen, moderate amounts of phosphorous and moderate to high amount of potash.

• Avoid nitrogen applications when the disease is active.

• Increase the height of cut.

• Increase the air circulation.

• Minimize the amount of shade.

• Irrigate turf early in the day to allow the foliage to dry as quickly as possible.

• Improve the drainage of the turf.

• Reduce thatch.

• Remove dew from turf early in the day.

• Fungicides are available to control the disease. [Consult your local County Extension Agent or Texas Cooperative Extension web resource-Ed.]