Red stripe produces narrow, sharply defined blood-red stripes, which may be short or extend nearly the length of the leaf. They are usually more prevalent on the younger, fully unfolded leaves; when conditions particularly favor the disease, however, infection spreads to the younger leaves and the growing point, often resulting in top rotting. The rot may extend into the mature portions of the stem to ground level, and is accompanied by a characteristic disagreeable odor. At one time the disease had some importance in several countries, but outbreaks now are infrequent and localized.

WEATHER markedly affects the severity of several diseases of sugarcane and sometimes may be a determining factor in their distribution. Red rot, for example, is important as a rot of seed cuttings only in the subtropical fringes of cane-growing regions in the Temperate Zone, while in the warmer Tropics, where uniformly high temperatures favor quick germination of the cuttings after planting, it is of minor importance.

Prolonged periods of wet, cool weather, such as commonly occur in Louisiana in winter, are unfavorable for the germination and growth of sugarcane, but they favor the development of fungi that cause seed rots and root rot. Growth of cane is slow between 60 and 70 F., temperatures which may occur for considerable periods during the winter months, but the fungi causing red rot and root rot are able to grow at those temperatures. Consequently, if the red rot fungus has gained entrance into the seed piece, it will continue to develop during cool periods when the cane is unable to grow. Likewise, the young rootlets produced by the cane during relatively warm periods may be attacked and largely destroyed by root-rotting fungi when their growth is checked by cool weather. For the same reason, freezing temperatures that kill leaves and shoots of young cane in winter or spring may cause more damage than merely killing the foliage itself. The cane plant loses what progress it has been able to make during favorable periods and because of exhaustion of food reserves may not be able to recover. But the disease organisms do not lose the progress they have made. Instead, they may continue to develop, destroying roots and further depleting food reserves in the seed cuttings. Therefore young cane that apparently has established a good stand may fail to recover following a freeze, and stands may die out during cool, wet spells in the spring even though a freeze does not occur.

During the growing season, particularly in summer, hard, dashing rains and winds aid in the spread of fungus and bacterial diseases that affect the leaves and stalks of cane. Dissemination of the stage of the red rot fungus Occurring on the leaf midribs is favored in this way. The leaf disease, brown spot, spreads most rapidly during the rainy summer months. The top-rotting diseases, pokkah boeng and red stripe are most severe during hot, humid weather. Eyespot, on the other hand, is favored by cool, humid Weather. The leaf diseases, brown stripe and mottled stripe, and the basal stem rot caused by Marasmius are more prevalent during dry weather.

CERTAIN DISEASES are more severe on some soils than others. It is common experience that root rot, red rot, and other seed-rotting diseases are more severe on the heavy clay soils than on the lighter-textured sandy soils. That largely is due to the higher moisture content of such soils resulting both from their physical structure and consequent greater moisture-holding capacity, but often also from their lower-lying position in the field, which makes drainage more difficult. Those conditions are unfavorable for germination of sugarcane seed cuttings and for root development, but they do favor root-and seed-rotting organisms.

ASIDE from those more obvious relationships, other biological factors are involved. Toxins may develop under the partial anaerobic conditions that often prevail in waterlogged soils. Antibiotic organisms affect sugarcane pathogens. Soil biological problems as related to sugarcane diseases have not been studied adequately. From practical experience, however, it is known that improvement of drainage results in increased yields, partly because there is less injury from diseases.

Chlorotic streak is more prevalent and severe on heavy, poorly drained soils than on lighter, well-drained ones. That fact has been observed in all countries where the disease has been studied, but the reason for it is not known. As we have no proof that the causal agent is soil-borne or that the activity of the one known vector (Draculacephala portola) is related to soil differences, it may be that the less favorable growing conditions for cane on heavy soils influences the metabolism of the sugarcane plant in such a way as to make it more susceptible to the disease.

THE SUGARCANE GROWER controls diseases mainly by replacing susceptible varieties with resistant varieties.

As a result of the ceaseless race between the growers and diseases, with the resultant never-ending varietal changes, probably no group of farmers is as conscious of varieties as are the growers of sugarcane. So spectacular have been some of the past achievements resulting from replacement of disease-susceptible with resistant varieties that to many growers a new variety still holds the possibility of a miracle and often, unfortunately, a fascination that cannot be resisted when the opportunity arises to obtain a new variety from other than approved sources.

Many diseases owe their spread from their native habitats to other countries to this search for new varieties. An unrecognized disease often is introduced in a variety that was brought into a country in an effort to overcome one already there. Before it was recognized as an infectious disease, mosaic was thus distributed over most of the sugarcane areas, and gummosis moved from the Western to the Eastern Hemisphere. Belated recognition of what was occurring resulted in the institution of quarantines in most countries, prohibiting the importation of sugarcane except through authorized government-controlled agencies. While those measures have greatly deterred the further distribution of diseases, as evidenced by the numerous interceptions that have been made in quarantine, they have not been entirely successful. Smut and leaf scald have become established in South America and chlorotic streak in Louisiana since the institution of quarantines.