1. FUNGI

Fungi are the largest group of plant pathogens. They can be thought of as plants which lack chlorophyll; but they are not plants, they are organisms in their own kingdom. Fungi obtain food from other living organisms or from decaying organic matter. They produce microscopic spores which can be compared to seeds of higher plants. The spores develop into threads (hyphae) which grow and branch into mycelium or other specialized structures (fruiting bodies).

Fungi enter plants through wounds, natural openings, or by direct penetration through the surface of the plant. The fungal mycelium grows through the plant and eventually produces more spores. These spores can then spread the disease to other susceptible plants. Some fungi have complicated life cycles which require more than one type of spore and/or more than one type of host plant to complete the life cycle.

Fungi are disseminated (spread) by airborne spores (wind currents), soil, water (move in irrigation water or rain splashes), seed, or by vectors. Vectors are agents that transmit diseases from one plant to another. Examples of vectors are: man, other animals, insects, tools, other microorganisms (fungi, nematodes, etc.), etc.

A. Ornamental Diseases Caused By Fungi (examples):

1. Powdery Mildew

Powdery mildew is the common name for the disease caused by several different fungi which produce a whitish, powdery growth on the surface of infected plants. A wide range of plants can be affected by powdery mildew, however each individual fungal species has a somewhat limited host range.

In general, powdery mildew fungi are favored by high humidity in the plant canopy (but are inhibited by water on the leaf surface) and warm temperatures. Cultural practices which increase air flow around plants, thus reducing the humidity in the plant canopy can help control powdery mildew. Additionally, protective fungicides are available for most plants, however timing of application is important in effective control.

a. Powdery Mildew of Rose

Powdery mildew of rose, caused by Sphaerotheca pannosa, is extremely common worldwide. The fungus attacks young, succulent foliage. The symptoms begin as slightly raised, blister-like, red areas on leaves. Eventually all infected above ground plant parts will develop a white powdery fungal growth.

S. pannosa overwinters in infected canes or buds and in fallen leaves. In spring, new shoots become infected from old mycelium from conidia (asexual spores) or from ascospores (sexual spores). Conidia and ascospores are disseminated to other susceptible hosts by air currents. The conidia and ascospores germinate and directly penetrate the plant.

The disease is favored by night temperatures between 58-62°F and day temperatures between 65-78°F. The fungal spores cannot germinate in free water, but germinate readily when the relative humidity in the plant canopy is high (97-99% at night and 40-70% during the day).

Powdery mildew is managed with good sanitation practices. Prune out all infected canes, remove fallen leaves, and destroy all infected plant material. Protective fungicide sprays can be used when weather conditions favor disease development. Plants already infected with the fungus can be treated with systemic fungicides which should help to reduce the spread and activity of the fungus. Be sure to check for registered materials and read and follow the label directions very carefully. When powdery mildew is known to be a severe problem, it is best to plant tolerant varieties.

b. Other common ornamental hosts of powdery mildew fungi: Euonymus, Photinia, Lilac, Pecan, Verbena, Crepe-Myrtle, Sunflower, Catalpa, Cotoneaster, Holly, Locust, Mesquite, Mulberry, Privet, Apple, Pear, Phlox, Zinnia, and Stone Fruits.

2. Verticillium and Fusarium Wilt Diseases

Verticillium spp. and some Fusarium spp. are common soil-borne fungi which cause wilt diseases in a wide variety of plants. While they are two distinctly different fungi, the types of diseases they cause and the plants they infect are similar. Plants infected with these fungi are wilted and chlorotic: In the case of Verticillium wilt, the plants may exhibit wilt or chlorosis on only one side of the plant. Infected plants also may have root rot. A helpful diagnostic symptom is the presence of vascular discoloration.

The fungi survive in soil and crop debris. They are spread by soil, water, wind blown dust, seed and infected plant material. Verticillium spp. generally do better at cooler temperatures than Fusarium spp.

Both of the fungi exist in many different genetic strains which vary in their aggressiveness and host range. Thus, a plant can be infected with a mild strain and exhibit chronic mild to moderate symptoms over a long period. Or, a plant can be infected with a severe strain and be killed within one growing season.

If the plant is infected with a severe strain, there is little that can be done to save the plant. If infected with a mild strain, good water and fertilizer management can help to slow disease development and reduce symptoms. However, the plant is still diseased and will eventually become weakened and should be removed and destroyed.

When replanting an area where a plant has died from this disease, it is best to choose plants that are either non-hosts, or are known to be tolerant to the disease. Always replant using disease-free seed and planting material. After planting, avoid injury to roots and crown and maintain good strong growth through proper water and fertilizer management.

a. Fusarium Wilt of Mimosa

Fusarium is a soil-borne fungus that invades trees through the roots. The fungus typically enters through wounds, but may also enter through direct penetration of the roots or plants weakened by abiotic stress. In addition to the typical symptoms listed above, this fungus causes infected Mimosa trees to ooze a frothy liquid from cracks and growth sprouts on trunks. Fungal spores produced on the exterior of the tree are easily washed off by rain or irrigation and can be moved long distance in surface water runoff. Control is dependent on sound cultural practices which maintains strong healthy trees. Mimosa trees should not be planted where trees have died from this disease. Some trees tolerant of this fungus include redbud, honey locust, and New Mexico locust.

3. Phymatotrichum Root Rot

Phymatotrichum root rot (also known as Texas root rot or cotton root rot) is caused by the soil-borne fungus Phymatotrichopsis omnivorum (PO). The fungus has an extremely wide host range affecting over 2,300 species of dicotyledonous plants (monocots are not affected, although the fungus has been found to grow and reproduce on some monocots without causing any disease). PO is limited geographically to parts of the United States (parts of Arizona, New Mexico, and Texas) and Mexico. Even within its geographical boundaries, the fungus is spotty in occurrence. The fungus is found in soils which are high in alkalinity and low in organic matter. Spread of the fungus is limited as it does not produce any viable spores, but spreads instead through root grafts between nearby plants.

Symptoms first appear during the summer when air and soil temperatures are high. The first evidence of the disease is a slight yellowing of the leaves. Quickly the leaves turn to a bronze color and begin to wilt. Permanent wilting can occur very rapidly; as little as two weeks from the first expression of disease. Plants infected with PO die rapidly with the leaves remaining firmly attached. In some cases, the tree wilts so quickly that there is little color change, though they become dry and brittle. A reddish lesion develops around the crown of trees killed by this fungus.

The fungus also produces signs on or near infected plants. Strands of fungal hyphae are produced on the surface of infected roots. These strands are usually visible with a good hand lens. When strands are viewed under a light microscope, cruciform (cross-shaped) hyphae unique to this fungus can be seen. Another sign is the formation of a white to tan colored spore mat on the surface of the soil around infected plants. Spore mats develop during periods of high moisture and are not always produced in New Mexico. Spores produced in spore mats have never been germinated, and are considered to have no function in the survival or infection by this pathogen. Therefore, spore mats do not spread disease, but are merely evidence of the presence of the fungus.

This disease is very difficult to control. The fungus survives over 12 feet deep in the soil reducing the effectiveness of soil treatments, such as solarization and fumigation. The fungus can be kept inactive by altering the soil environment (reducing alkalinity and increasing organic matter). Kept in mind that the following treatment must be applied very quickly after the first sign of disease. Additionally, the treatment will need to be applied every year in order to prevent reoccurrence of the disease. This treatment is no guarantee of control and is expensive and labor intensive.

Steps to control PO:

1. Heavily prune back infected trees or shrubs.

2. Loosen soil underneath the plant out to the drip line.

3. Cover ground with 2" of composted manure.

4. Cover manure with ammonium sulfate (1 lb to 10 sq. ft.).

5. Cover ammonium sulfate with soil sulfur (l lb to 10 sq. ft.).

6. Soak area with water till water penetrates the soil to a depth of 3-4 ft.

7. Also treat any adjacent susceptible trees or shrubs even if there are no symptoms.

Avoiding areas known to be infested with the pathogen is the best control measure.

4. Rust Diseases

Rust is the common name for the disease caused by several different fungi which produce dark-colored spore pustules on the surface of infected plants. A wide range of plants can be infected by rust fungi, but -most individual rust fungi have a very limited host range. Rust fungi may have up to five different spore stages in their life cycle. Rusts may be autoecious (having only one host) or heteroecious (having two hosts). Heteroecious rusts need both hosts to complete its sexual life cycle, although inoculum can build on one host through asexual reproduction. When two hosts are required to complete the life cycle, one is often considered the "economic host" (the desirable plant) and one is called the "alternate host". In some cases, such as apple-cedar rust, both hosts are "economic hosts".

The presence of spore pustules is a ten-tale sign of rust infection. Spores produced in rust pustules can be carried by wind currents up to several hundred miles. Spores are also moved short distances by wind, insects, rain, and animals. Additional symptoms include, leaf and stem cankers, stunting, yellowing, galls, and a general unsightly appearance of the infected plant.

Rust fungi may be controlled by the integrated use of several different management practices. Rusts with two host may be reduced by eliminating the alternate host (if one of the hosts is undesirable). Removing infected bedding plants or other annuals will help to reduce spread in the garden. Depending on the host, tolerant varieties may be available. Lastly, contact and systemic fungicides, such as triforine, maneb, sulfur, ferbam and zineb, may be registered for use on several host plants. Check chemical labels for appropriate use of these materials.

5. Black Spot of Rose

Black spot of rose, is a fungal disease caused by Diplocarpon rosea. The disease develops in moderate temperatures when moisture is present on the leaf surface. General leaf chlorosis and circular, well defined, black spots on leaves are the most common symptoms. The disease is similar to powdery mildew in that it overwinters in canes and in fallen leaf debris. The fungus germinates in favorable conditions and is spread to susceptible hosts by splashing water or airborne spores. Sanitation is important in the control of black spot. Rake and destroy fallen leaves and prune out infected canes. Look for tolerant varieties if black spot is a common problem in your area. Also some fungicides, such as captan, chlorothalonil, and triforine, are registered for control of powdery mildew. Check fungicide labels for complete instructions before using any chemicals.

6. Sooty Mold

Sooty mold is a term used to describe the black sooty fungal growth on many trees and shrubs. Several different fungi can cause sooty mold. These fungi are generally not parasitic to the plants they grow on, but are growing on honeydew produced by insects (aphids, scale, and mealy bugs). Sooty mold is common in warm, humid weather. The fungi appear on leaves, stems or fruits as a superficial, black growth. The fungi do not penetrate the host tissue and can be wiped off with a damp cloth. Although sooty mold fungi are not pathogenic, they do create a problem when: the growth of the fungi, becomes dense reducing the amount of light which reaches the green leaves. This reduced light limits carbohydrate production by the plant and weakens the growth of plant. The most effective means of controlling sooty mold is managing honeydew producing insects.

B. Foliar Turfgrass Diseases Caused by Fungi (examples):

1. Dollar Spot (causal agent: Sclerotinia homeocarpa)

Most all turfgrass species are susceptible to dollar spot. The disease results in circular, sunken patches (up to 6 cm in diameter) in the turf. Chlorotic, water-soaked lesions which become white with age occur on individual plants. These lesions may be surrounded by a tan to reddish brown margin. The lesions may be circular or hourglass in shape or may enlarge to cover the entire leaf surface. The tips of infected grass blades dieback. With heavy dew, a white cottony, fungal growth occurs over the surface of the infected grass.

The fungus, S. homeocarpa, survives as mycelium in infected plants or plant debris. It spreads from one area to another by movement of infected plant material by equipment, people, animals, water or wind.

The disease occurs from late spring through fall, during periods of high humidity in the leaf canopy. Warm (60-86F), humid days and cool nights which result in heavy dew favor disease development.

2. Downy Mildew or Yellow Tuft (caused by: Sclerophthora macrospora)

All turfgrass species are susceptible to downy mildew. Turf affected by downy mildew exhibits an overall spotty appearance. The diseased patches are small (1-10 cm in diameter) and chlorotic. Individual plants are stunted and thickened or have broadened leaves and shortened roots. The whole plant becomes yellow over time. A white, downy growth may appear on leaf surfaces during cool, wet periods.

The fungus, S. macrospora, survives as mycelium or oospores (sexual resting spores) in infected plants or plant debris. It is spread by movement of infected plant material by equipment, people, animals, water, or wind. Swimming spores produced by the fungus can move short distances in water and contribute to the enlargement of diseased areas.

The disease occurs from late spring through fall, when the leaf surface is wet. The disease is favored by poorly drained soils and heavily watered areas.

3. Powdery Mildew (caused by: Erysiphe graminis)

The most common turf species affected by powdery mildew are bluegrass and fescue . The disease results in large areas of the turf appearing as if they were dusted with powder. The individual plants are infected with isolated colonies of whitish fungal growth, which rapidly enlarges to cover the entire leaf surface. Infected leaves eventually turn yellow then tan to brown as they die. Older leaves are more susceptible to attack than the young succulent growth. Dark fungal fruiting bodies called cleistothecia may form in mycelial mats on the surface of leaves.

The fungus, E. graminis, survives as mycelium in living, infected plants or as cleistothecia embedded in plants or plant debris. The fungus is spread by airborne spores which can move great distances in air currents.

The disease occurs in the spring and fall. It is favored by temperatures between 60 and 72° F, high humidity, and cloudy periods. The disease is most severe in shaded areas with poor air circulation.

4. Rust (caused by several species of rust fungi)

All turfgrass species are susceptible to rust fungi. Turfgrass patches infected with rust are thin and weak, and are tinted red, brown or yellow in color. The individual plants exhibits light yellow flecks on infected leaf blades and stems. The yellowish area of infected leaves enlarge and elongate parallel with the leaf or stem axis. Rust pustules on the surface of infected plants expose colored spores.

Rust fungi survive as mycelium and spores in infected plants or plant debris. The fungi are spread by airborne spores which can move short and long distances in air currents.

The disease occurs from spring through fall on grass which is growing slowly under stressed conditions. Disease development is favored by warm (68-86° F) temperatures, low light intensity, and moist leaf surfaces. After infection, disease spreads rapidly with high light intensity, dry leaf surfaces and high temperatures (above 85° F).

5. Copper Spot or Zonate Leaf Spot (caused by: Gloeocercospora sorghii)

Most turfgrass species are susceptible to copper spot, however, the disease is most severe on bentgrass. Affected turf areas will exhibit scattered, roughly circular diseased patches, approximately 2-7 cm in size, that are salmon-to-copper colored. Individual plants have small red to brown lesions with may coalesce to blight the entire leaf. During warm, wet weather, infected leaves are covered with mycelium and fruiting bodies containing salmon-colored spores in a gelatinous matrix.

The fungus, G. sorghii, survives in infected leaf debris. It is spread by the movement of spores by equipment, people, animals, or water.

The disease occurs from late spring through fall and is favored by warm, wet weather. Grass growing under excessive nitrogen fertilization or on acidic soil is more susceptible to the disease.

6. Snow Molds (caused by several different fungi)

All turfgrass species are susceptible to at least one type of snow mold. The symptoms vary somewhat depending on the grass species and the fungus, but typical symptoms include; spots to large patches which are yellow, white, gray, brown, or pink in color.

Snow mold fungi survive as mycelium or fruiting bodies in infected plants or plant debris. They are spread by leaf to leaf contact, and usually require wet leaf surfaces for infection.

The disease occurs from winter to early spring. Snow molds are associated with low temperatures, frozen soil, snow cover or several freeze/thaw cycles during the winter. Snow molds are also favored by moisture from snow, frost, rain, and dew.

Control of snow mold is usually accomplished by application of fungicides (labeled for the disease) before the first lasting snow and during periods of snow melt.

C. Foliar and/or Root Turfgrass Diseases Caused by Fungi (examples):

1. Anthracnose (caused by: Colletotrichum graminicola)

All grass species are susceptible to anthracnose, however the disease is most severe on bluegrass and bentgrass. Symptom development is highly dependant on the environment, but scattered chlorosis or irregularly shaped chlorotic patches ranging from a few centimeters to a few meters in size, is characteristic of infected turf. Individual plants have water-soaked lesions on the leaves or stems. The lesion eventually turns reddish brown and the entire leaf turns yellow, then tan to brown as the leaf dies. Tiny black fruiting bodies may appear on infected stems and leaves.

The fungus, C. graminicola, survives as mycelium in plant debris. It is spread by movement of spores by equipment, people, animals, water, and wind.

The diseases occurs any time of the year, but is most severe during the summer months. Disease development is favored by high humidity and leaf wetness. Grass which is under stress, particularly from high temperatures and drought, are particularly susceptible to this disease.

2. Helminthosporium-like Diseases (melting out diseases)

Melting-out diseases, caused by several different species in the genera, Bipolaris, Curvularia, Drechslera, and Exserohilum, are among the most common turf diseases which occur in New Mexico. All turfgrass species are susceptible to one or more of these pathogens. Symptoms of this disease are variable depending on the fungal species causing the disease, however, some general symptoms include overall thinning or decline of turf followed by irregular patches 5 centimeters to 1 meter in size. The individual turf plants are hard to find in infected areas as it appears as if the turf has "melted" away. The appearance on individual plants can be in the form of brownish-green to black lesions, dappled yellow and green patterns on the leaves, or elongated water-soaked lesions with a yellow halo.

The fungi survive in infected plants or plant debris and are spread by the movement of spores by equipment, people, animals, water, and wind.

The diseases occurs from spring through fall. Diseases caused by Bipolaris are favored by either wet or dry conditions and temperatures between 68-86° F. Curvularia diseases are favored by wet conditions and high temperatures (above 86° F). Drechslera diseases are favored by wet conditions and cool temperatures between 55-65° F.

3. Fusarium diseases (caused by: Fusarium spp.)

All turfgrass species are susceptible to disease caused by Fusarium spp. Diseased turf develops sunken, circular to irregular shaped patches between 2 and 30 centimeters in size. Patches may develop a "frog’s eye" symptom - dead circles with live grass in the center. Individual plants will have black to brown "dry rot"; of the roots, ‘crowns, rhizomes, and stolons. Infected leaves start out light green in color and rapidly fade to tan. White to pink mycelium develops on infected grass during periods of high temperature and moisture.

The fungi survive as mycelium in infected plants or plant debris. They are spread by movement of spores by equipment, people, animals, water and wind.

The disease occurs from late spring through summer and is favored by high temperatures and drought stress. Susceptibility increases in grass with excessive nitrogen or unbalanced fertilizer applications.

4. Pythium Diseases (caused by: Pythium spp.)

All turfgrass species are susceptible to Pythium diseases. While the symptoms are somewhat variable, the disease are typified by an overall decline in the turf area. This decline may be gradual or rapid, depending on the environmental conditions. Small areas of declining turf may coalesce to cover large areas. Individual plants have dark, water-soaked lesions. The leaves turn yellow, then tan. Roots of infected plants rot. In the early morning when dew is present, a white, cottony growth may appear on the grass surface.

The fungi survive as oospores in infected plants or plant debris. They are spread by movement of infected plant material by equipment, people, animals, and water. Swimming spores move short distances in water and contribute to the enlargement of individual areas.

The disease occurs anytime during the growing season. Disease development is favored by hot (86-95° F), wet weather with night temperature above 68° F. Susceptibility increases in dense turf and in turf growing in alkaline conditions.

5. Brown Patch (caused by: Rhizoctonia solani)

This is one of the most common turf diseases in New Mexico. All turfgrass species are susceptible to brown patch. Although the symptoms are highly variable, the disease is characterized by small to large rings or patches of dead grass. Individual plants have small to large, irregularly shaped lesions with a distinct dark brown margin. Infected leaves become chlorotic, then brown with age. Dark brown sclerotia, (masses of mycelium with a hard shell) may develop at the base of infected plants.

The fungus, R. solani, survives in soil and in infected plants and plant debris. It is spread by leaf to leaf contact, and by movement of infected plant material by equipment, people, animals, water and wind.

The disease occurs from spring through fall and is favored-by warm (70-90° F), wet conditions. Dense, highly fertilized, frequently watered grass is more susceptible to the disease.