D. Root Turfgrass Diseases Caused by Fungi (example):

1. Summer Patch (caused by Magnaporthe poae)

Summer patch is common on bluegrass and fescue, but also has been found on ryegrass and bentgrass. It is a common disease on golf greens and is often identified when bluegrass is killed and seemingly unaffected bentgrass grows into diseased patches. Before the causal agent, M. poae, was identified, the disease was thought to be part of the Fusarium blight complex.

The symptoms first appear as small (3-8 cm), circular patches of slow-growing, thinned, or-wilted turf. The diseased area may increase in size to 30-60 cm in diameter. Affected leaves rapidly fade from grayish green to a light straw color during sustained host weather. Infected roots, rhizomes, and crowns turn dark brown as they are killed. Microscopic examination of these tissues reveals a network of sparse, dark brown to black hyphae.

The fungus survives as mycelium in plant debris or perennial host tissue and is spread by aerification and dethatching equipment as well as by transport of infected sod.

Infection occurs in the spring when soil temperatures stabilize between 65 and 68° F. Symptoms develop during hot (86-95° F) rainy, weather or when high temperatures follow periods of heavy rainfall. The disease is more severe when turfgrass is maintained under conditions of low mowing height and frequent, light irrigation.

Fairy rings are characterized by circles, rings or arcs of by dark green, faster growing turf areas. In many cases., the diseased areas die-out as the disease progresses leaving dead area surrounded by dark green growth. Rings can vary in size during the year and can appear or disappear throughout the growing season. The dark green areas may be surrounded by mushrooms, toadstools, or puffballs which are fruiting bodies of the fungi which cause the disease. These fruiting bodies are excellent signs of the fungi and can be very diagnostic.

The fungi which cause fairy rings live in the soil and decomposing thatch layer. They are spread from one area to another by the movement of infected plant material or infested soil by equipment and wind blown spores.

The diseases occurs from spring to early summer, and the fruiting bodies generally appear in the late summer (during the summer rainy period). The fungi are favored by light textured soils, low fertility, and drought.

Fairy rings are difficult to control. Fungicides are rarely effective. The affected turf must often be removed including a significant amount of soil. The area must then be prepared and resodded. Removing fruiting bodies before they mature can help to reduce the severity of the disease.

Slime molds are not disease causing agents as they do not penetrate and infect plants. However, they do cause harm to plants by covering the leaf surface and reducing the amount of light reaching the plant surface. Slime molds are pinhead-sized white, gray, or purplish-brown fungi, which are slimy in appearance and texture. They appear in patches 2 to 60 cm in diameter) during periods of warm, wet weather. They usually disappear within 2 weeks after the favorable conditions are gone. Heavily thatched turf is more susceptible to disease. Slime molds can be washed off of plants with a hard stream of water, or swept off with a broom.

Bacteria are single cell microorganisms that lack chlorophyll. They obtain food from living or dead organic matter. The average size of a bacterium is less than 5 microns. Bacteria reproduce by division of the cell into two equal parts. Bacteria can divide very rapidly: Under the proper environmental conditions one bacterium can produce over 1 million identical cells in less than 1 hour.

Bacteria enter plants only through wounds or natural openings on the plant surface. Once inside the plant, bacteria multiply rapidly and break down the plant tissue creating a watery mass. Bacteria are only apparent to the naked eye if clumped together to form a colony (bacterial ooze).

Bacteria are spread by splashing rain (waterborne), in soil, in windblown dust, by vectors (man, tools, equipment, insects, etc.), and infected planting materials (seed, transplants, cuttings, etc.). Diseases caused by bacteria are generally favored by high air and soil moisture. Because of this, bacterial diseases are few and somewhat rare in arid regions.

A. Diseases of Ornamentals and Vegetables Caused by Bacteria (examples):

Bacterial spot- and blight diseases affect all above ground parts of plants: leaves, stems, blossoms, and fruit. A wide variety of ornamentals and vegetables are susceptible to spot and blight diseases. Spot diseases are characterized by necrotic, circular or angular shaped spots. In some diseases, the spots are surrounded by a yellow halo and/or dead leaf tissue may fall out of spot leaving holes in the leaf (known as a "shot hole"). Bacterial blights are characterized by a continuous, rapidly advancing death of infected organs.

The bacteria overwinters in infected and healthy tissue, in or on seeds, in plant debris, and in soil. The organism is spread by rain, leaf to leaf contact, insects, cultivation, and tools. Water soaking of tissue caused by heavy rains predisposes plants to infection.

Bacterial spots and blights are managed with good sanitation practices; removing and destroying infected plant parts and frequent cleaning of tools and other equipment. Copper fungicides and antibiotics can be useful for high value crops, such as in orchard or nurseries. In vegetable crops, look for tolerant varieties, and rotate crops around the garden.

Fire blight is a bacterial disease caused by Erwinia amylovora. It affects plants in the rosaceae family, most notably apple, pear, rose, pryracantha, photinia and cotoneaster. Infected plants exhibit blighted branch tips which are black and look like they have been scorched by fire. Another symptom is the development of a shepherd’s-crook on young, vegetative shoots. The bacterium overwinters as cankers and symptom less infections in leaf and flower buds. In the spring, bacteria oozes from infected cankers and is spread by water, pruning tools and insects (primarily bees) to nearby blossoms. Ideal conditions for infection and development of disease are rain (or high humidity) and temperatures between 75-85° F.

Cultural practices can be highly effective in the management of fire blight. Infected plant parts should be pruned, cutting at least 6" below the disease margin (margin between healthy and diseased tissue), and destroyed. When pruning infected trees, it is advisable to dip pruning shears in a 10% bleach solution or in 70% alcohol in between cuts. Avoid over fertilization (especially with nitrogen). Provide adequate, but not excessive water. Copper fungicides and antibiotics can be effective sprays, however timing is critical and improper use can lead to phytotoxicity (from the copper chemicals) or development of resistance in the bacterial population.

Bacterial canker (also referred to as "gummosis") is caused by several different species of bacteria. It affects stems, branches, twigs, leaves, buds, flowers, and fruit. The disease occurs on a wide range of hosts, including stone fruits, apple, pear, lilac, rose, tomatoes and small grains. Symptoms include: splits in trunks or stems, necrotic areas in the woody tissue, and sunken cankers which may be soft, leathery, or scabby in appearance. Some cankers exude a slimy or gummy substance. The bacteria over winter in perennial cankers, in buds, plant debris and in or on seed. The disease is spread by rain, runoff water, cultivation, tools and infected plant material.

Bacterial canker is best controlled by good sanitation practices. This includes pruning and destroying infected plant parts and maintaining strong, but not excessively vigorous growth. Avoid planting any material which is suspicious in appearance. Copper fungicides or antibiotics may help in reducing spread when many similar plants are in close proximity (e.g. home orchards). Caution should be taken in using copper fungicides as improper use may lead to plant injury and effective use is dependent on proper timing of applications.

Crown gall, caused by the soilborne bacterium Agrobacterium tumefaciens, has a wide host range among woody and herbaceous plants. The bacterium enters roots or stems near ground through wounds created by cultural practices, insects, etc. The bacterium can also infect plants above the crown by pruning with infested cutting shears.

Once inside susceptible hosts, the bacterium stimulates host cells to enlarge causing tumorlike galls. The galls impede the movement of water and nutrients in the plant. Reduced transport of water and nutrients causes chlorosis, stunting, slow growth, and a general decline in the health of the plant. Some plants infected with crown gall will continue to grow seemingly unaffected while others decline over time until they have to be removed.

Once infected, there is little that can be done to help the plant other than providing adequate water and nutrients. Well managed trees are less likely to go into a rapid decline. When planting susceptible hosts, use disease-free nursery stock. Avoid injury to roots and crown at planting and during cultivation of turf or other plants nearby.

3. VIRUSES AND VIROIDS

Viruses consist of nucleic acid (RNA or DNA) surrounded by protein (coat protein). Viroids consist only of nucleic acid (RNA only). Both are so tiny they can only be seen with an electron microscope. They do not carry out respiration, digestion, or other metabolic functions, therefore, they are not living organisms. Viruses and viroids cannot grow or multiply outside the host cell, but cause the plant to transform host plant components into more virus or viroid particles.

Viruses and viroids are transmitted from one plant to another by vectors (so they are biotic disease agents). Common vectors include man, insects, budding, grafting, nematodes, fungi, seed and/or pollen. The virus or viroid particles enter plants through wounds created by the vector.

Symptoms of these types of diseases are nonspecific and can look like symptoms caused by many other disease agents. General symptoms include stunting, yellowing, curling or twisting of leaves and stems, distorted leaves and fruit, mosaics, mottles, and ring spots.

A. Diseases of Ornamentals and Vegetables Caused by Viruses (examples):

1. Rose Mosaic Virus

Rose mosaic virus occurs worldwide. The symptoms are highly variable depending on the variety and the environment, however chlorotic bands or rings, vein clearing, and general mosaics are common. Symptom development on only a portion of a plant is common. Infected plants have decreased vigor, poor flower production, and are more susceptible to winterkill. The virus is transmitted through vegetative propagation and pollen. There is no control for a plant infected with the virus. Infected plants should be removed and destroyed.

Beet curly top virus (BCTV) is common in arid and semiarid regions on a wide host range (affects over 300 plant species). The virus is transmitted (vectored) by the beet leafhopper (Circulifer tenellus). Some of the more common hosts include tomatoes, peppers, cucurbits, potatoes, beans, spinach, geranium, nasturtium, petunia, stock, and zinnia. Weeds are important survival hosts for the virus and the vector. Weed hosts include; Russian thistle, sow thistle, London rocket, pigweed, purslane, knotweed, and lamb's-quarter.

Symptoms vary somewhat depending on virus strain and host plant, yet there are some common characteristics among infected plants: Overall chlorosis, upward curling of leaves, thickening of leaves and stems, stunting, deformed fruit, and reduced fruit production. On some hosts, such as tomato and pepper, leaf veins on the underside of the leaves may turn purple. Although viruses usually do not kill their host plants, young seedlings attacked by BCTV may die.

The only means of disease transmission is by leafhoppers. Leafhoppers feeding on infected plants will rapidly (1 minute) acquire the virus. The virus then circulates through the insect and can be transmitted to a susceptible plant after as little as 4 hours. The leafhopper then remains infective for the remainder of its life, although the effectiveness of transmission is decreased when the insects do not continually feed on infected plants. The virus is not passed on to progeny, however, young leafhoppers which develop on infected hosts will quickly become carriers of the virus. As far as is known, the virus has no negative or positive affect on the insect. Leafhoppers over winter in winter weeds. Mild winters and large populations of winter weeds are two important factors in BCTV epidemics.

There are no chemicals available for the control of viruses, but several cultural practices can help to reduce or eliminate infections. Good sanitation practices, including weed and insect control and removing infected or suspect plants, are essential in limiting the occurrence of the disease. Home gardeners may also consider planting susceptible hosts, such as tomatoes and peppers, in a slightly shaded part of the garden, as leafhoppers prefer to feed in sunny locations. However even shaded plants will become infected when leafhopper populations are high. Placing netted cages over susceptible hosts (particularly when young) may help to prevent infection. The netted material should be small enough to prevent leafhoppers from getting through the material. It is also important that the cages be large enough that the plants do not touch the netting. When plants mature, the cages should be removed. At this stage, the plants is less susceptible to the virus. Although there is little tolerance known to BCTV in commercial varieties, this is an active area of research and new varieties may soon be released which possess a level of resistance to the virus or the leafhopper.

Tomato spotted wilt virus (TSWV) is an important disease of many vegetables and ornamentals in temperate and subtropical regions of the world. In New Mexico, it is particularly troublesome in greenhouses, but can easily move to gardens on infected plant material or carried by its vector (thrips). The virus has a tremendously wide host range including tomatoes, peppers, celery, lettuce, spinach, potatoes, peanuts, begonias, geranium, nasturtium, impatiens, petunia, snapdragons, verbena, stock, and statice. Like curly top, TSWV has many weed hosts which help it survive from one season to the next. Common weed hosts include curly dock, field bindweed, lamb's-quarters, pigweed, morning glory, jimsonweed, and nightshade.

Symptoms of TSWV are numerous and varied. Some fairly characteristic traits of infected plants are bronzing and yellowing of leaves, leaf spots, leaves become distorted, and petioles curl downward creating a wilt-like symptom although the plants retains its turgor pressure. Other symptoms include dieback of the growing tips and dark streaking of the terminal stems. Infected plants may develop a one-sided growth habit or may be completely stunted. Infected plants produce little or no fruit. Fruit that is produced exhibits symptoms, such as necrotic streaking, raised bumps, chlorotic spots or ring spots, uneven ripening, and deformation.

TSWV is transmitted from infected to healthy plants by at least 9 species of thrips. Thrips transmit the virus in a persistent manner, which means that once the insect acquires the virus it can transmit the virus for the remainder of its life. The virus is not passed from adult to egg; however, progeny that develop on infected plants will quickly pick up the virus and become effective disease vectors.

Controlling the disease is difficult. The wide host range, which includes perennial ornamentals and weeds, enables the virus to successfully over winter from year to year. In landscape situations, controlling thrips does not always translate into reduced disease. This is probably due to the fact that large populations of thrips may fly or be blown into treated areas from non-treated area nearby. Controlling thrips, is somewhat more effective as a disease control measure in greenhouses. Control of thrips may be obtained with pyrethroids, carbamates, chlorinated hydrocarbons, organophosphates, and insecticidal soaps. However great care should be taken to avoid repeated use of any one type of chemical as thrips in the treated population may rapidly build resistance to the material. Rotating the insecticide class is the best approach to insect control. Pesticide registrations are constantly changing so it is important to read the label for legal uses and follow all label instructions carefully. In greenhouses, thrips populations may be reduced by covering all openings (doors, vents, etc.) with a fine mesh (400 mesh) screen.

While elimination of disease may not be possible, the incidence and severity of the disease may be reduced by several cultural practices. It is .important to start with virus-free plants. Do not purchase or plant any plant which exhibits symptoms such as described above. Remove any infected or suspect plants from the greenhouse, garden and landscape. Control weeds. Efforts are underway to breed cultivars with good horticultural characteristics that also exhibit tolerance to the virus.