Reports on Plant Diseases

Stripe smut, caused by the fungus Ustilago striiformis, and flag smut, caused by Urocystis agropyri, are two leaf or foliar smuts widely distributed in the world and destructive to many turfgrass species. Smut fungi weaken the grass host making the plant easy to kill when under severe stress. Plants infected with flag smut are killed more readily than are plants infected with stripe smut. Flag smut is often more prevalent in early spring, while stripe smut generally predominates in late spring and early autumn. These smut fungi grow systemically throughout a grass plant, and both smut fungi may infect the same grass plant at the same time and even the same leaf. Once infected, a plant remains so for life. Infected plants are often weakened and invaded by other organisms.

Leaf smuts, together with high temperatures and drought, cause grass plants to exhibit stunted growth, a brown to blackish brown appearance, a general decline, and early death. Dead patches of grass often appear in heavily infected turf during midsummer; weed invasion soon follows (Figure 1).

The stripe and flag smut fungi infect about 100 species of turf and forage grasses, both cultivated and wild (Table 1). A number of highly specialized varieties and pathogenic races of these fungi are restricted to certain cultivars and species of grass. The diseases occur most commonly on annual bluegrass and Kentucky bluegrass (especially on the cultivars Delta, Geronimo, Merion, Newport, Park, Rugby, and Windsor). Creeping bentgrass, colonial bentgrass, redtop, fescues, common timothy, several wild-ryes and wheatgrasses, orchardgrass, perennial or English ryegrass, and quackgrass are also commonly infected. Leaf smuts are favored in locations having excess thatch, frequent irrigations or rains during spring and summer, turf that is 3 years or more old, a pH below 6.0, and where susceptible grass cultivars are grown.

Back to Top

Symptoms

Due to the killing of individual plants and the distinct upright growth of infected plants, the turf, in an overall view, appears clumpy and patchy. Smutted plants are most noticeable during cool weather in the spring and autumn, appearing pale green to slightly yellow or brown, stunted, and more upright than healthy plants. Single plants, or irregular patches up to one foot or more in diameter may be affected.Short to long, narrow, and yellow-green streaks (sori) develop between the veins in infected leaves and leaf sheaths. These streaks soon become silvery to dull gray and extend the entire length of the leaf blade and sheath (Figure 2). The grass epidermis covering the streaks soon ruptures, exposing blackish brown, dusty masses of smut spores (teliospores). After dispersal of the spores, the leaves soon split and shred into ribbons, turn brown, curl from the tip downward, turn light brown and die. In addition, leaves of infected bluegrass and bentgrass plants tend to remain stiff and erect rather than lax and spreading. The symptoms are usually most evident in middle to late spring and autumn, when temperatures average 50 to 65 F (10 to 18 C). Affected plants do not tiller as profusely or produce as many rhizomes or stolons as healthy plants, nor do they develop as extensive a root system.

Smutted plants are often difficult to find during hot, dry weather because a large percentage of such plants often die during summer droughts (Figure 1). Both smut fungi decrease leaf turgor and water potentials of infected plants under drought stress. Once infected, grass plants will rarely, if ever, recover unless properly treated with a systemic fungicide.

Under close mowing, both smut fungi produce identical symptoms. Positive diagnosis can be made only through a microscopic examination of the teliospores produced by the smut fungus. The spores of Ustilago striiformis are single cells, round to elliptical in shape and covered with prominent spines (Figure 3). The spores of Urocystis agropyri are smooth and roundish and are composed of one to four, dark reddish brown, fertile cells (teliospores) surrounded by several smaller, empty, or sterile cells (Figure 4) forming a spore ball.

The stripe and flag smut fungi overwinter and oversummer as dormant mycelium in the meristematic tissue of crowns and nodes of infected plants and as dormant teliospores in or on grass debris, living plants, and soil. The teliospores are carried to the thatch and soil by many agents, including wind, rain, shoes, mowing, watering, raking, dethatching, coring, and other turf maintenance practices. Spores may also be transported on the seed. These teliospores may lie dormant in soil for up to 3 years (4 years on stored seed) before they germinate.

When suitable conditions occur in the spring and autumn, a teliospore germinates to produce mycelium on which minute spores (sporidia) may be borne, although this is apparently rare on certain grasses, such as Kentucky bluegrass and creeping bentgrass. Each sporidium then germinates and forms a germ tube. When the germ tubes of opposite mating types fuse (conjugate), an infection hypha forms that penetrates susceptible host tissue directly. Invasion may occur through the coleoptile of seedling plants and actively growing (meristematic) tissues produced by the lateral or auxiliary buds on the crowns, rhizomes, and stolons of older plants that come into contact with germinating spores. Once inside the grass plant, smut hyphae develop systemically in the direction of plant growth, with new leaves, tillers, rhizomes, and stolons becoming infected as they form. The mycelium continues to grow within developing tissues.

GRASSES SUSCEPTIBLE TO BOTH STRIPE AND FLAG SMUT

GRASSES SUSCEPTIBLE ONLY TO STRIPE SMUT

fairway crested wheatgrass (Agropyron cristatum)
thick-spike wheatgrass (A. dasystachyum)
Agrostis castellana
spike redtop (A. exarata)
redtop (A. gigantea)
A. humilis
A. hyemalis
A. microphylla
autumn or brown bentgrass (A. perennans)
Ross redtop (A. rossae)
rough bentgrass (A. scabra = A. geminata)
European beachgrass (Ammophila arenaria)

mountain hairgrass (Deschampsis atropurpurea)
tufted hairgrass (D. caespitosa)
Elymus semicostatus subsp. Striatus = Agropyron striatum
Siberian wild-rye (E. Sibiricus)

American beachgrass (A. breviligulata)
broom-sedge (Andropogon virginicus)
vernalgrass (Anthoxanthum sp.)
Arctagrostis latifolia
oatgrass (Arrhenatherum sp.)
Avena pubescens
American sloughgrass (Beckmannia syzigachne)
quaking-grass (Briza sp.)
plains reedgrass (Calamagrostis montanensis)
C. pickeringii
Scribner reedgrass (C. scribneri)
timber oatgrass (Danthonia intermedia)

GRASSES SUSCEPTIBLE ONLY TO FLAG SMUT

goatgrass (Aeglilops sp.)
Baker wheatgrass (Agropyron bakeri)
bearded wheatgrass (A. X subsecundum var. andinum)
thin grass (Agrostis diegoensis)
beardgrass (Andropogon sp.)
oatgrass (Arrhenatherum sp.)
California or mountain brome (Bromus carinatus = B. aleutensis)
hairgrass (Deschampsia sp.)
Elymus aristatus
basin or giant wild-rye (E. cinereus = E. condensatus)

E. scabrus = Agropyron scabrum
Elytrigia sp.
chewings or red fescue (Festuca rubra = F. rubra subsp. rubra, F. rubra var. lanuginosa, F. rubra var. prolifera)
fowl mannagrass (Glyceria striata = G. nervata)
Hierochloe sp.
Hordeum brachyantherum = H. boreale
ryegrass (Lolium sp.)
California melic (Melica imperfecta)
big bluegrass (Poa ampla)
Wheeler bluegrass (P. nervosa = P. wheeleri)
bread or common wheat (Triticum aestivum = T. sativum,
T. vulgare, T. timopheevii)