Gray leaf spot is common on foxtail millets in the United States; it is a minor, but sometimes destructive, leaf Spot on many grasses. Severe spotting progresses to a blighting or blasting of the foliage. The causal fungus, Piricularia grisea, is like the fungus that causes the blast disease of rice. No control has been suggested for gray leaf spot. Because the grass is an annual plant, crop rotation should help reduce the disease.

Downy mildew, Sclerospora graminicola, is probably the most serious leaf disease on foxtail millet in the United States. Affected plants are dwarfed because elongation of culms is retarded. Excessive tillering from the crown and development of branches from the axillary buds along the culm are characteristic. Leaflike malformations of the floral bracts and failure of kernel development are other common symptoms. A downy mass of spores is common on infected plants in humid areas.

Leaf killing and browning are followed by splitting and shredding of the invaded leaf tissues, especially as plants approach maturity. Excessive proliferation of buds and seed heads combined with little or no kernel development causes a serious reduction in yield if infection is high.

Control of downy mildew is difficult in places where those crops are grown continually over large areas because of general soil infestation and wind-borne spores. It is difficult also when the crop is sown in areas where the wild Setaria viridis is infected.

Formaldehyde, sulfuric acid, and organic mercury compounds are the best seed treatments.

DROPSEEDS (Sporobolus species) are attacked by 36 organisms. Leaf diseases include leaf rust, stem rust, ascochyta leaf spot, bacterial leaf spot, powdery mildew, phyllachora tar spot, selenophoma speckle, cercospora leaf spot, septoria leaf spot, stagonospora leaf mold, and false smut.

False smut or head mold, caused by Helminthosporium ravenelii, has a striking appearance. The fungus grows over the seed heads and covers the affected parts with a velvety, brownish-olive mantle, which later becomes black and crusted. Head mold is so common on Sporobolus indicus and S. poirehi that they are called smutgrasses.

It would be a good thing to treat seed to prevent spread of false smut on infected seed, but that cannot control the disease in infested areas because of contamination by wind-borne spores. Burning might help. The use of resistant varieties seems to be the best solution in places where the disease does great damage.

NEEDLEGRASSES (Stipa species) suffer from 50 or more diseases. The more important are septoria leaf spots, selenophoma leaf and stem spots, stagonospora leaf blotch, and scolecotrichum brown stripe.

Brown stripe disease is caused by the fungus Scolecotrichum graminis. Young leaf infections show water-soaked, circular or oval lesions, which are olive gray in the morning when they are wet with dew and dull gray when they are dry. The spots become brownish purple to ocher with gray centers. They tend to form streaks as the leaves slowly die. The spore-bearing bodies of the fungus can be seen as prominent, black dots arranged in parallel rows.

Many grasses are affected with brown stripe. The fungus causes one of the most important leaf spot diseases of timothy, orchardgrass, blue- grasses, tall oatgrasses, redtop, and needlegrasses. Early maturity of timothy and some other grasses is often forced by loss of leaves killed by the fungus.

Careful burning of dead grass reduces the spores for infection of new leaves. The relationship of infected wild grasses as sources of infection of domestic grasses needs study. Varieties resistant to the disease are needed in the needlegrasses, tall oatgrass, orchardgrass, and many others.

IT IS HARD TO CONTROL leaf diseases of the range grasses. Extensive application of chemical dusts or sprays is out of the question because of the danger of poisoning livestock. Such materials generally are too expensive for use on forage plants even when land is used intensively for pastures. The cost of treating the many low-producing acres on ranges would be prohibitive.

Crop rotation, clipping, and deep plowing obviously are not feasible for range land. Early or late grazing might remove some infected leaves, but that is not wholly effective because animals avoid badly diseased and dead leaves.

Seed treatment may be helpful in preventing introduction of diseases carried on the seed to new areas in the initial reseeding of range land. Spores of many disease fungi are wind-carried into the new plantings from wild stands, however, and nullify much of the value of seed treatment.

Fire is an effective but dangerous means of destroying diseased leaves. By exercising extreme care and utilizing fire breaks and other precautions to prevent uncontrolled range fires,. burning can be a cheap method of reducing initial infection by some diseases. There is, of course, a loss of organic matter and the risk of killing valuable perennial grasses, so that undesirable plants might take over the range lands.

All in all, then, the best way to control most range grass diseases is to use resistant plants. Development of disease-resistant grasses requires exact information on behavior of the disease-producing organisms so that tests for resistance can be performed and understood. Such information is not available for many of the leaf diseases.

A good start has been made, however. Research by many workers has shown which species can be grown in various sections of the country. The identity of the organisms causing many grass diseases has been determined. Disease problems are being further clarified. Prospects are good that more and more adapted, high-yielding, disease-resistant grasses will become available eventually to increase forage on range lands.

JOHN R. HARDISON, a graduate of Washington State College, obtained his doctorate in plant pathology at the University of Michigan. He has been engaged in work on diseases of forage crops since 1942.