Sugarcane and Its Diseases

E. V. Abbott.

Sugarcane is native to or is grown in some 50 countries or political units within approximately 400 north latitude to 32 south. It is an important crop for thousands of small farmers with only a few acres. It also is the basis of large plantation enterprises. It is native to the Eastern Hemisphere, but since the Second World War the Western Hemisphere has produced about 55 percent of the world's cane sugar.

Sugarcane is commonly thought of as a thick-stemmed, tall grass, cultivated in tropical regions for the production of sugar from its juices. That conception is correct only in a limited sense. It does not take into consideration the numerous wild forms of the sugarcane genus Saccharum that are not thick-stemmed, high in sucrose content, or grown in tropical regions.

Even the present commercial sugarcanes often disappoint the person who sees them for the first time, particularly in the southern United States, where, instead of the large-stemmed, broad-leaved sugarcane of the textbook, the visitor finds a plant with narrow leaves and stalks of more slender girth a plant that, while possibly lacking some of the physical attractiveness of its noble-type ancestors, has gained the vigor, disease resistance, and adaptability to other elements of its environment that are essential in this section. It is, in fact, a new plant, created to meet specific requirements through the blending by cross-breeding of the so-called noble and wild forms of the genus Saccharum.

The known forms of sugarcane are classified into five species: Saccharum officinarum the traditional sugarcanes, which have thick, soft stems, broad leaves, and attractive appearance and therefore are commonly termed "noble" canes; S. barberi, the slender-stemmed canes of northern India; S. sinense also slender-stemmed types, native to China; S. spontaneum, native to continental and insular Asia and to parts of Africa, a diverse group of grassy forms, which do not have direct commercial use for sugar production, but which are important sources of vigor and disease resistance in breeding; and S. robustum, wild forms of great vigor, indigenous to New Guinea and some adjacent islands.

Disease epidemics are known to have occurred in Mauritius and Reunion in the 1840's and in Brazil in the 1860's. Complaints of "degeneration" were heard elsewhere. Since the true nature of the difficulties was not understood at the time, however, the outbreak of sereh disease in Java in the early 1880's, which seriously threatened the industry there, stands out as of greater historical importance because of its influence on the future of sugarcane breeding and disease investigations. Sereh disease, presumed to be caused by a virus, forced a complete change in field practices, including the adoption of an expensive system Of growing seed cane in elevated areas and the abandonment of ratoons. Satisfactory control was not attained Until the Black Cheribon variety was eventually replaced with more resistant ones.

Sereh did not become widely distributed over the sugarcane-growing areas of the world. But another virus disease, mosaic, which was first observed in Java in the early 1890's, was carried to practically every cane-growing country before its true nature and seriousness Were suspected. In most of them it assumed epiphytotic proportions that forced changes in varieties and caused enormous economic loss. In Louisiana, where it was superimposed on varieties already declining from the combined effects of root rot and red rot, it brought the industry to the brink of ruin in the 1920's. No other sugarcane disease has attained the universal importance of mosaic; yet, while it can still be considered a potential danger wherever it occurs, it has become one of the first major sugarcane diseases to be brought under a satisfactory degree of control in most countries through the development of resistant or tolerant varieties.

A third trouble, which caused widespread concern to the sugarcane world beginning in the late 1890's and continuing into the present century, was attributed to what was variously termed root rot, root disease, or root disease complex. In Java, the West Indies, Hawaii, Louisiana, and elsewhere, the declining yields and crop failures of the period were attributed in varying degree to root disease, but with less than universal agreement as to the specific cause. In the West Indies in particular, some of the losses at first attributed to root disease were later found to be due to red rot.

Species of the fungus genus Marasmius, to which the noble cane varieties are susceptible, were at first believed to be the main cause of root disease, but Rhizoctonia and other fungi were also blamed by some workers. Those fungi, however, are much less aggressive parasites than a member of the genus Pythium (P. arrhenomanes), which later was found to be the chief cause of root rot. Nevertheless, while all of the factors concerned in the growth difficulties of the period were not correctly diagnosed at the time, it was concern over root disease that stimulated the search for resistant varieties and the investigations that led to a better understanding of this and other diseases and their control.

BESIDES SERER AND MOSAIC, five other diseases of sugarcane assumed to be of virus nature have been identified Fiji disease, streak, chlorotic streak, dwarf, and ratoon stunting.

There have been unverified reports of the occurrence of sereh in the Philippines, Formosa, and India, but it has not assumed commercial importance outside Java. The disease has no certain diagnostic symptoms. Its effects vary widely on different varieties. A composite picture would include the extreme stunting of affected plants, with the arrested growth of successive shoots in the stool and a bunch-grass appearance; sprouting of the lateral buds on affected canes into leafy shoots; and excessive production of aerial roots at the nodes. No insect vector has been determined.

Fiji disease, named for the islands in which it was first observed, was identified in 1910, although it is believed to have been present there for at least 20 years before it was recognized as a disease. It caused serious loss to the industry of the islands before being brought under control by the use of resistant varieties. It has been definitely identified only in New Guinea, Queensland, and the Philippines, outside of Fiji. The disease causes extreme stunting of affected plants. It has one distinctive diagnostic symptom the elongate galls it produces on the veins of the under surface of the leaves. Its insect vectors are members of the genus Perkinsiella.

Streak is also of limited geographic distribution. Its known occurrence is confined to South Africa, Egypt, India, and the island of Mauritius. It is characterized by the production of many narrow, elongate, sharply defined white streaks on the leaves. Its principal effect on the plant is reduced growth, with consequent loss in tonnage. While causing important loss to the Uba cane predominant in South Africa at the time of its discovery, losses have been greatly reduced by the substitution of more resistant clones. Its insect vector is Cicadulina mbila.

A comparative newcomer in the list of sugarcane diseases is chlorotic streak, first observed and described in Java in the late 1920's. It has since been found in Hawaii, Australia, Mauritius, Puerto Rico, Louisiana, and British Guiana. It is most prevalent and severe in cane growing or, heavy, poorly drained soils, where it may markedly reduce germination, growth, and ratooning. Its only known insect vector is the leafhopper Draculacephala portola.

Two other diseases, presumed to be caused by viruses, dwarf and ratoon stunting, have been described in Queensland. Dwarf is not known to occur elsewhere, but what is apparently the same as the ratoon stunting disease was identified in the United States in 1952. This disease is of particular interest because it has no well-defined symptoms other than the retarded growth of affected plants. Because of this insidious nature, its presence was not suspected until experiments showed that the growth stunting could be transmitted. It is transmitted in infected cuttings, by knife cuts, and by inoculation of seed cuttings with juice of infected plants.

IN ANY PLANT that is propagated vegetatively, as sugarcane is, with the seed pieces consisting of succulent stems rich in carbohydrates, it is not surprising that rots of the seed cuttings are serious causes of poor stands. At each node of the stem is an axillary bud, which, when planted under required conditions of moisture and temperature, germinates to produce a new plant. Also at the node is a narrow band of rudimentary root buds from which the rootlets develop as the shoot bud germinates. The young plant depends on these roots, called seed roots, until it has produced sufficient stem tissue of its own for the permanent root system, or shoot roots, to develop. The seed roots are only temporary, but they serve the plant during a critical period between germination of the bud and establishment of the system of shoot roots.

Commercial practice with respect to the use of planting material varies in different countries. In the Tropics, where the high temperatures and adequate moisture usually favor a quick germination of the buds, the immature upper parts of the stem are used for planting. The more mature lower part of the stalk is left to be milled. In the subtropical sugarcane areas, such as Louisiana, where low temperatures following planting often mean a delay of several weeks before the new plants are established, the whole stalk is used as seed. It may be planted without segmenting, but more often is cut into shorter sections before planting, or, as in Louisiana, after being laid down in the furrow. In the Tropics, where there is little delay in germination of the buds after planting, seed-rotting diseases are much less important than in the subtropical fringes of the sugarcane-growing area, such as Louisiana, India, South Africa, and Queensland.

Of the seed-rotting diseases, red rot is one of the most serious. It rivals mosaic in its nearly universal distribution in the sugarcane world. First described in Java in 1893, it has since been identified in all countries where sugarcane is important.

A second widely distributed seed-rotting disease is caused by Ceratostomella paradoxa. Commonly it is termed pineapple disease because of the characteristic odor of the rotting cuttings, which is like that of decaying pineapples. The interior of affected seed pieces becomes sooty black. Eventually only the vascular bundles are left as fibrous strands in the hollow, blackened core. In contrast to red rot (which is favored by excessive soil moisture), pineapple disease is most destructive when cane is planted in soil that is too dry. Another major difference is that the red rot fungus is not soil-borne and infection occurs before planting, whereas the pineapple disease fungus lives in the soil. That fact makes the pineapple disease more susceptible to control through the use Of protective fungicides. It may also cause a rot of the stalk of growing cane, but that is not common.

Seed rots have also been ascribed to several other fungi, including Ceratostomella adiposum (black rot), Fusarium species (probably the same organism described as Cephalosporium sacchari as the cause of wilt in India), Cytospora sacchari, and Phytophthora species (in Louisiana).