Red Rot of Sugarcane

E. V. Abbott.

Red rot is a serious disease of sugarcane in the southern United States. It causes a rot of seed cuttings that commonly results in faulty stands of plant cane or sometimes complete failures, reductions in stands of stubble or ratoon crops because of the rotting of the underground parts of the stem from which the crops arise, and annual losses of sucrose in mill cane from infection of the stalks that usually follows injury by the sugarcane moth borer, Diatraea saccharalis.

Besides the loss in tonnage from reduced stands of cane, red rot lowers the amount of recoverable sugar at the factory because of the inversion of sucrose in the stalk, which is accompanied by lowered purity of the juice and other untoward chemical changes.

Because it is most destructive as a rot of seed cuttings, red rot causes little injury to seed cane if conditions favor the early germination of the buds after planting, as in the Tropics. In the subtropics, however, such as India, South Africa, and Queensland, where considerable time may elapse between the time of planting and the establishment of new plants, it frequently reduces stands badly. As a rot of mill cane, on the other hand, it may be more important in the tropical areas, where the longer growing season gives the fungus more time in which to spread through the stalks between the time of infection and milling of the cane.

Red rot was first described in Java in 1893. Shortly thereafter it was identified in the West Indies, where it was considered at first to be a cause of the then prevalent root disease. Within the next 20 years its occurrence was recorded in Queensland, India, Hawaii, and Louisiana. It now is one of the most widely distributed of the diseases of sugarcane.

Red rot has been a major cause of the decline of several varieties of sugarcane in the Southern States. First identified in Louisiana in 1909, it doubtless was a factor in the general downward trend in the average yield that began early in this century in Louisiana. It was one of the causes of the failure of the Louisiana Purple and D-74 varieties in Louisiana in the 1920's and of those and other noble-type varieties in the other Gulf States. It forced the discontinuance as a commercial cane of the variety P. O. J. 2714 in southern Florida.

The noble varieties were replaced in Louisiana with hybrids from Java, one of which, P. O. J. 213, became the leading commercial variety in the State by 1931. Classed as resistant to red rot when it was released, it suddenly failed from red rot in the early 1930's. In the sirup-producing districts of the Gulf States, Cayana 10 and P. O. J. 213 became the most popular replacements of the older canes, but both eventually succumbed to red rot. Some of the other varieties that have succeeded them as important commercial canes have also declined from red rot, but they were tested more rigidly before they were released to growers, and their potential weakness in this respect was recognized at the time of their release.

THE FUNGUS that causes red rot may infect any part of the sugarcane plant. Its principal importance is as a rot of the stalk of standing cane, of seed cuttings, or of the stubble pieces remaining in the ground after the cane is harvested. It produces long lesions on the leaf midribs. The lesions usually cause no serious injury to the plant but are important in the life history of the disease because they are sources of the spores that cause infection of the stalk.

Red rot often cannot be told on external examination of the stalk unless it has rotted the interior so completely as to cause the rind to lose its natural bright color and to look dull. Plants so affected may be detected by the yellowing, shriveling, and dying of the upper leaves. More certain identification may be made by splitting the stalk or seed cutting. Then one recognizes the disease by the reddening of the normally white or creamy-white internal tissues and cross-barring of the reddened area with occasional white or light patches. Unless the cross bars are present, identification of red rot may be uncertain without microscopic examination or culturing of the fungus. Almost any sort of wounding causes a reddening of the stalk tissues next to the wound, but when red rot is present the characteristic discoloration usually extends considerably beyond the point of origin. In advanced stages of rotting, the interior of the stalk darkens and the tissues shrink, leaving a cavity, which may be filled with the mycelium of the fungus.

The lesions on the leaf midribs are dark or blood red, and may occur as short, discontinuous blotches or as long ones that extend nearly the length of the leaf. The centers become straw-colored with age and are later covered with the black, powdery masses of the spores of the fungus.

The fungus causing red rot is commonly known by its imperfect stage, Colletotrichum falcatum, although Physalospora tucumanensis is the perfect stage of the fungus. If a fairly large number of isolates of the fungus obtained from different cane varieties or geographic areas are studied on artificial culture media, considerable variation in the type of growth and color of the fungus colony usually is seen. Some isolates or races are light gray and form a loose cottony colony. Others are dark gray and form a restricted velvety colony. Others are intermediate in those respects. If they are inoculated into stalks of sugarcane, the fact that they differ also in pathogenicity their ability to infect and rot the stalks will be seen.

It is this variability of the fungus that makes it appear that some sugarcane varieties are unstable in their resistance to the disease. Often a new variety, when it is released to growers, may be resistant to the races of the fungus then prevalent. If a race that is virulent toward that variety is present or appears later, however, it may build up on it and eventually cause serious injury. We have evidence that that occurred with the variety P. O. J. 213 in Louisiana in the early 1930's. Some growers believe this change in predominance of races of disease organisms indicates an inherent change in the sugarcane variety with respect to its disease resistance. The real explanation, though, is to be sought in changes in the prevailing populations of the disease organism.

THE INFECTIONS of the leaf midrib provide the means of dissemination of the disease during the growing season and the source of inoculum for stalk infections. The infections appear in Louisiana in the late spring and continue to develop on new leaves as they are produced during the summer.

On the midrib lesions the fungus produces an abundance of spores, which are carried by wind or splashed by rain to other leaves and plants. Heavy dews and rains wash the spores down the leaf blade to the attachment of sheath to the stalk, where the spore-laden moisture may be held for some time in contact with the nodal region of the stem. The spores may also be washed down the stalk, where they cause infections through the tunnels made by the moth borer.