Plant Diseases: http://www.ipm.ucdavis.edu/PMG/menu.disease.html

Dry rot

Dry rot—Stromatinia gladioli Dry rot attacks plants such as daffodils, freesias, and gladiolus. The initial infection attacks corms either in storage or in the soil. After the initial infection, decay spreads up to the leaf bases. Dry rot causes leaves to yellow and die. Plants are stunted and fail to bloom. Leaf bases or stems rot near the soil and tissue appears shredded. Very small black sclerotia are imbedded in dead tissue. Bulbs develop dark brown, sunken lesions with raised margins. Solutions Dry rot is favored by cool, wet soil. Don't grow in infested soil during cool weather. Use pathogen-free bulbs or plants. Provide good soil drainage. Avoid overwatering. Remove infested plants and bulbs.

Verticillium Wilt

Wilting, chlorosis, and stunting of plants. Leaves often have a marginal necrosis. Vascular discoloration, which is characteristic of this disease on other hosts, may not be always present in artichoke plants. Diseased plants produce smaller buds, and in severe cases, buds become discolored and dried, and the plant collapses. Roots exhibit the characteristic vascular discoloration of this disease. There is no effective treatment for verticillium wilt. For affected vegetables, remove and dispose of the plant; don't compost it. For landscape plants, prune out affected branches and dispose of them immediately. Do not use infected wood for chips for landscape mulch. Keep plants vigorous by providing trees with proper irrigation, fertilizer, and other appropriate care to promote new growth and increase their chance for survival. Chronic branch die-back may develop in surviving trees; prune out any dead wood. Regularly inspect for possible hazards; affected trees may need to be removed. Where Verticillium wilt has been a problem, plant only resistant species. Soil solarization before planting may be effective. Table 1. Trees and shrubs susceptible to Verticillium Ash Dogwood* Plum Azalea Elder Redbud Barberry, Japanese Elm Rose Buckeye, Ohio Honeysuckle Russian olive Catalpa Linden* Smoke tree Cherry, other stone fruits Locust, black Spirea Coffee tree, Kentucky Magnolia Sumac Cork tree Maple Viburnum Currant and gooseberry Oak, pin and red (rare) Wigela Table 2. Trees and shrubs resistant or immune to Verticillium Apple Hawthorn Oak, white and bur Arbovitae Hickory Pear Beech Honeylocust Pine Birch Hophornbeam Poplar Butternut Juniper Serviceberry* Dogwood* Larch Spruce Fir Linden* Sycamore Gingko Mountain ash Walnut Hackberry Mulberry Willow *Some plant species are listed in both tables (linden, dogwood). The resistance or susceptibility will depend on the cultivar and the strain of Verticillium present in the soils.

Alternaria blight

Alternaria blight—Alternaria spp. Alternaria blight is a disease on carnation. It causes tan spots or blotches with purple margins to occur on leaves, flowers, or stems. Lesions may be around the stem base. Solutions Use good sanitation. Remove infected plants and debris. Avoid overhead irrigation. Water in the morning so that plants dry quickly.

Leaf spot diseases

Leaf spot diseases Many fungi cause leaf spots on different hosts. Spots may vary from small discrete dots and raised areas to irregular yellow or brownish patches that cover much of the leaf surface. Leaves may fall off the tree if the problem is severe, but these pathogens rarely cause long-term damage to trees. Similar spots can be caused by bacterial pathogens, insects and mites, or abiotic factors on some plants. Solutions In most cases, infections can be tolerated. Remove fallen leaves and debris promptly. Many of the pathogens are favored by moisture, so avoid overhead sprinklers and irrigate early in the day so that the foliage dries more quickly. Generally, fungicide treatment is not warranted.

Bacterial soft rot

Bacterial soft rots, leaf spots, blights, wilts—Erwinia, Pseudomonas, Xanthomonas spp. Bacterial soft rots affect many plants including begonia, carnation, daffodil, geranium, impatiens, and zinnia. Soft rot bacteria cause infected tissue to turn brown, become mushy, and develop an unpleasant odor. Stem tissue turns brown and deteriorates near the soil. Plants grow slowly and seedlings collapse. Bacterial spots often start out as tiny water-soaked areas on leaves, stems, or blossoms. Spots or blotches turn dark gray or blackish as they enlarge and sometimes have yellow borders. Initial spots are circular but may become angular and coalesce and cause plant tissue death or necrosis. Cankers may form on stems. Under wet conditions, infected tissue may exude brownish masses of bacteria. Dead tissue may tear out, leaving holes and a ragged appearance. Pathogens causing spots, blights, and soft rots can also cause vascular wilt if the infecting bacteria become systemic. Aboveground plant parts yellow, droop, wilt, and die. Solutions Use disease-free cuttings, corms, and other stock. Avoid planting too deeply. Provide good drainage. Do not overwater and avoid overhead irrigation. Keep foliage dry and provide good air circulation. Don't crowd plantings. Bacteria commonly infect through wounds, so avoid injuring plants. Use good sanitation. Regularly inspect plants for disease and remove infected plants immediately. Some cultivars are more susceptible to infections than others. Seek information on resistant cultivars and consider planting them.

Clubroot—Plasmodiophora brassicae

Clubroot is a problem on plants such as cabbage, broccoli, alyssum, and nasturtium. During initial stages of clubroot, above ground symptoms may be absent. Foliar symptoms include stunting, yellowing, and wilting. Extensive galling, swelling, and distortion of the roots and hypocotyl are the main symptoms of the disease. Clubroot is common in soils where Brassica spp. plants have previously grown. ldentification - Clubroot may often be confused with nematode damage. Aboveground symptoms of both disorders are similar--wilting or stunting of leaves. Life cycle - The fungus persists in soil for many years. Solutions - Clubroot is most common in acid soils. Add lime annually to affected soils below pH 7.2. Provide good drainage. Minimize the spread of the pathogen by using pathogen-free transplants. Avoid planting plants where other infested plants in the mustard family have grown, such as broccoli and cabbage. Solarization will also give control.

Downy mildew

Downy mildew—Plasmopara (=Peronospora) spp. Downy mildew causes problems on various plants including alyssum, carnation, pansy, poppy, snapdragon, sunflower, and sweet pea. Downy mildew causes soft and fluffy gray, purplish, or light brown sporulation on the underside of leaves and sometimes on stems or buds. Pale yellow areas or irregular purplish red to dark brown necrotic lesions are sometimes visible on the upper surface of infected leaves. Solutions Provide good air circulation and maintain low humidity. Avoid wetting foliage; use drip instead of overhead irrigation where feasible. Dig out infected plants as soon as possible to reduce the spread of disease. Several fungicides can prevent infection of healthy tissue, and fungicide use may be necessary to prevent damage to susceptible plants if conditions are good for disease development.

Fasciation

Fasciation affects many plants, including alyssum, aster, carnation, chrysanthemum, geranium, impatiens, Marguerite daisy, nasturtium, petunia, and primrose. Fasciation is an abnormal flattening of stems, often appearing as if several adjoining stems have fused. Fasciated plants have short, swollen clumps of distorted shoots. Leaves growing from distorted stems are abnormally abundant and undersized. Distortion often develops at the plant base, and crowns may appear galled. Sometimes only new terminal growth is affected. The cause of most fasciations is not understood. Some may be genetic and others may be caused by bacterial or viral infections. Fasciation bacteria survive on infected plants and debris. They spread in water and through wounds. Solutions Control bacterial fasciation primarily through good sanitation and use of pathogen-free plants. Avoid injuring the base of plants, especially when plants are wet. Keep the base of plants dry. To control fasciation to all potential causes, do not propagate or graft symptomatic plants. Remove and dispose of infected plants, or prune and dispose of distorted tissue and do not propagate from those plants.

Fire blight

Fire blight causes blossom clusters to wilt and collapse in late spring. A brownish, sticky exudate is produced from diseased tissue. The tips of infected young succulent shoots curve into a characteristic shepherd's hook. Management relies on maintaining trees in the proper range of vigor, applying blossom sprays of antibiotics or copper, and most importantly, promptly finding, removing, and destroying blight strikes. Remove holdover cankers by cutting back to healthy wood. This disease can be difficult to control.

Flower blight—Ovulinia azaleae

Flower blight—Ovulinia azaleae Flower blight causes small round to larger spots on flowers. Flowers collapse, become soft, and cling to leaves or stems. The fungus that causes flower blight is favored by cool, wet weather. Spores are spread by splashing water. Solutions Remove and dispose of diseased blossoms. Remove debris from the base of plants or pots in order to reduce inoculum. Prune to improve air circulation and remove lower branches where infections first occur. Do not use overhead sprinklers. In rainy areas, applying a fungicide such as triforine prior to budbreak may help prevent this disease.

Fusarium wilt

Fusarium wilt—Fusarium oxysporum Fusarium wilt affects relatively few woody ornamental species but can kill certain hosts, including albizia, date, palm, hebe, and pyracantha. Most forms of Fusarium oxysporum attack only herbaceous plants including aster, carnation, chrysanthemum, dahlia, and freesia. Fusarium wilt causes foliage to yellow, curve, wilt, then turn brown and die. Fusarium wilt symptoms often appear first on one side of a plant. Older leaves usually die first in infected plants, commonly followed by death of the entire plant. Plants infected when they are young often die. Cutting into infected wood may reveal that vascular tissue has turned brown, often all the way from the shoot to the soil line. Cross-sections of basal stems may reveal brown rings. Masses of spore-bearing stalks are sometimes visible on dead tissue and may look like small pink cushions. Solutions Fusarium wilt results from infection through roots by hyphae that germinate from long-lasting survival structures in the soil. Plant with species from different genera rather than with plants previously infected there by Fusarium. Choose resistant cultivars if available. For herbaceous species, plant on raised beds. Provide proper sanitation and cultural care to reduce plant susceptibility to infection and damage. Avoid overwatering and provide good drainage. Avoid applying excessive fertilizer. Chronic branch dieback may develop in surviving trees; prune out any dead wood. Regularly inspect for possible hazards; affected trees may need to be removed. Soil solarization before planting may be effective.

Powdery Mildew

Powdery mildew is a common disease on many types of plants and is prevalent under the diverse conditions found in many areas of California. Different powdery mildew fungi cause disease on different plants. These fungi tend to infect either plants in the same family or only one species of plant. IDENTIFICATION AND DAMAGE You can recognize this disease by the white, powdery mycelial and spore growth that forms on leaf surfaces and shoots and sometimes on flowers and fruits. Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, water on plant surfaces for extended periods inhibits germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60° to 80°F and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95°F may kill the fungus. MANAGEMENT Avoiding the most susceptible cultivars, placing plants in full sun, and following good cultural practices will adequately control powdery mildew in many situations. Some ornamentals do require protection with fungicide sprays if mildew conditions are more favorable, especially susceptible varieties of rose and crape myrtle.

Rust fungi

Rusts—Many species Rusts are easily recognized by the dry, brown, orange, purple, reddish, or yellowish spore masses or pustules that form on the lower leaf surfaces. Rusts infect many plants, including aster, carnation, geranium, iris, lily, pansy, primrose, snapdragon, sunflower, and sweet pea. The upper surface of heavily infected leaves can become spotted or turn yellow or brown, and infected leaves may curl, wither, and drop prematurely. Severely infected plants may be stunted. Some rust species cause tissue swellings, galls, or cankers, especially on woody plant parts. These rusts can cause stem dieback and, rarely, can kill the entire plant. Solutions Rust fungi infect under mild, moist conditions. Reduce infections by minimizing the length of time that foliage is wet. Avoid overhead watering, which favors rust spore germination and spread. Alternatively, water early in the day so that plants dry more quickly. Use good sanitation. Remove and destroy affected plants or plant parts as soon as they appear. Prevent excess humidity, provide good air circulation, and don't crowd plants. Some plants are resistant to rust. Consider using these to avoid rust problems. Fungicides, including neem oil, applied at the first signs of infection can also prevent serious damage from most rust fungi.

Scab

Scab—Pseudomonas gladioli Scab is a disease of gladiolus caused by bacteria that penetrates the corm and then moves up the stem base. Scab causes corms to develop brown to black, irregular or round, sunken spots with shiny brittle, varnishlike bacterial exudates on the surface. Leaf bases have red or brown specks. Solutions Scab is favored by poorly drained, wet, warm soils. Provide good soil drainage. Don't overirrigate or overfertilize. Control chewing insects in soil that injure corms. Remove and destroy infected plants.

Botrytis

Smoulder—Sclerotinia or Botrytis narcissicola Smoulder infects bulbs, such as daffodil, both in storage and in the ground. When infected bulbs start to grow, symptoms are see on foliage. Smoulder causes brown, streaked lesions to appear first on leaf tips. Leaves infected on the inner edge may curl. Diseased tissue may have woolly gray growth and small black sclerotia, especially near the soil. Solutions Smoulder is favored by cool, wet weather. Provide good drainage. Dig out infected plants. Avoid planting bulbs in areas where infected plants have grown previously.

Viruses

Viruses Most flowers are susceptible to infection by one or more plant viruses. Viruses can slow plant growth and change the appearance of foliage, flowers, and fruits. Virus-infected leaves can become spotted, streaked, mottled, distorted, or stunted. Veins may lose their color or develop outgrowths. Flowers can be dwarfed, deformed, streaked, or faded, or they can remain green or develop into leaflike structures. Viruses usually infect through a wound. Many are transmitted by invertebrates, such as aphids and thrips, that feed on plant juices. Mites, nematodes and fungi can also transmit viruses. Viruses can spread in vegetative plant parts, such as cuttings from infected stock plants and in bulbs, corms, and rhizomes. Identification Solutions Viruses rarely kill woody plants, but can dramatically alter plant appearance, reducing the value. Herbaceous ornamentals and certain vegetables are generally more susceptible to serious injury or death from viruses, especially when plants are young. Most plants infected with a virus cannot be cured. Dig out and destroy virus-infected plants. Prevent spreading viruses by using good sanitation and cultural practices. Control nearby weeds that serve as reservoirs for viruses and insects. Use only virus-free plant material and consider growing virus-resistant cultivars if available.