Smuts That Parasitize Grasses

George W. Fischer.

Nearly 140 different species of smuts attack approximately 300 species of grasses in the United States.

The common name smut derives from words meaning to besmirch or smudge and refers to the presence of sooty, dirty, black, or brownish masses on the affected plants.

All the smuts are plant parasites. Most of them occur on grasses and cereals. As with the rusts, the average person is much more familiar with the destruction wrought by smuts on cereals than on grasses. Familiar examples of destructive cereal smuts are bunt or stinking smut of wheat, oat smuts, barley smuts, sorghum smut, and boil smut on corn. In everyday practice, the name smut is used to designate the disease as well as the fungus responsible for the disease, although the latter is more correctly spoken of as the smut fungus.

The smut fungi have much less complicated life histories than do the rust fungi and generally are easier to control. The familiar dusty black smut masses are made up mostly of millions of tiny cells or groups of cells spores, which serve the same purpose to the smut fungi as seeds do to the seed-bearing plants, namely, for reproduction and dissemination.

Some smuts destroy the flowering structure. Others are restricted only to certain parts of it. Some are confined almost exclusively to the stems of grasses. Others produce galls or tumorlike structures in various parts of their host plants.

As with the rust fungi, many smut fungi exhibit a remarkable degree of specialization not only to certain species of plants but also to certain varieties or strains within those host species. Likewise, there are often strains or races of the smut fungi to contend with. The common stinking smut, for example, has nearly 30 known strains or races, each capable of attacking different varieties of wheat and different strains or varieties of wheatgrasses and related grasses.

The smut fungi are not entirely obligate parasites as are the rust fungi. In fact, some smuts can easily complete their entire life cycle on an artificial medium if it contains the nutrients essential for growth. Some smuts apparently persist indefinitely in the soil or in old manure piles. They are fully capable, however, of attacking their host plants when the plants are available under the requisite conditions for infection.

The smut fungi have a more adverse effect directly (and perhaps indirectly) on their hosts than do the rust fungi. The smuts that attack all or parts of the flowering structures generally destroy the seeds entirely. The leaf smuts and the stem smuts, while only occasionally involving the flowering structures, do nevertheless generally suppress these structures and likewise result in a more or less complete loss of seed on affected plants. The smuts that attack the vegetative structures (that is, the leaf smuts and stem smuts) have a decidedly weakening effect on their host plants and make them more susceptible to other sinister factors in their environment.

Many of the grass smuts are seed-borne. The wind carries millions of smut spores, which become lodged in or on the developing seeds of healthy Plants. When the seeds germinate, the smut spores also germinate and infect the young seedlings. Then the Plants that arise from the seedlings are smutted.

Some of the smut fungi are not limited to seedling infection. Apparently they can infect any succulent or rapidly growing part of their host plants and thereby make them smutty in time. The time between infection and the appearance of smut varies rather widely. In the stripe smut of grasses, for example, seedlings often show smut within 6 weeks of the time the smut-contaminated seed is planted. Other smuts are not evident until their hosts head out. One of the stem smuts, to be described later, requires 2 to 4 years after infection before smut actually appears.

Upper left: The ergot fungus has replaced a seed in the orchardgrass flower head. Upper right: Fruiting bodies of a fungus growing from seed of perennial ryegrass. Lower left: Spores and mycelium of a fungus magnified by the lens of a microscope. Lower right: Variability in progeny from a cross of two corn smut fungus cultures.

Spores of the smut fungi generally are much more durable than are those of the rust fungi. It is hard to maintain viability of rust spores for more than a year; usually it is much less than that. In the case of the smut fungi, it is unusual for viability to be maintained for less than a year. For most smuts viability is maintained for two to several years, especially if humidity is low. Some smuts have been known to retain at least some viability for 25 years.

The cultivation of grasses seems to have much to do with the development of smut in them: Often cultivation practices, including harvesting and threshing, disperse the smut and contribute toward an increase in the amount of smut in succeeding crops. Any of the grass smuts therefore is a potential threat to the welfare of its grass host if the grass comes under cultivation.

Some of the grass smuts are the same as those that are destructive parasites of our cereal crops.

SEED SMUTS or bunt (Tilletia species) are terms used here to designate the smuts of grasses in which the normal seeds are replaced by smut "balls." The balls retain somewhat the shape of the normal seed and are even encased by the seed wall; actually, though, the inside of the seed is a solid mass of spores.

About 25 of these grass seed smuts occur in the United States. Several have actual or potential importance in cultivated grasses. One of the potentially most important is the one that on wheat is commonly called stinking smut or bunt. Three species of fungi are involved in it, including what is commonly known as dwarf smut. These smuts are well known on wheat because of the very extensive losses they have caused for a long time the world over. Besides wheat, tall oat-grass and several of the wheatgrasses are known to be susceptible, including crested wheatgrass, slender wheatgrass, and intermediate wheatgrass. Care has to be taken to keep those grasses from falling prey to the cereal smuts.

The dwarf bunt is soil-borne. It has thus far presented a knotty problem of control except when resistant varieties are used. The other two species, being mostly seed-borne, may be controlled by seed treatment and the use of resistant varieties.

Some of the bentgrasses are susceptible to another of the seed smuts, especially on the east and west coasts where considerable smut infestation is sometimes encountered in harvested seed crops. The life history of this smut is not known.

Various other groups of grasses, for example the bromes, fescues, wild-ryes, hairgrasses, velvetgrass, and others, are prey to similar seed smuts.