Sugarcane Section

Sugarbeet Section

Entomology Section

Pathology Section

SBS Jabban

Sugar Crops Pathology

Pathology section of SCRI, Mardan, mainly conducts research on diseases and their control. Besides, the technical staff actively participates in providing data on severity and intensity of various diseases on sugarcane and sugarbeet germplasm during variety development process.

A description of common diseases of sugarcane is given below. The ensuing material has been adapted with thanks from J. C. Comstock and R. S. Lentini, Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.

1. Sugarcane Diseases

1. Ratoon Stunting Disease (RSD)

Ratoon stunting disease (RSD) is considered by many to be the most important disease affecting sugarcane production worldwide. It can cause a 5 to 15% loss in crop yield without the grower even knowing his fields have been infected. The disease is caused by a bacterium. RSD has no easily recognized external symptoms, only stunting of growth that may not always be apparent in the field. Furthermore, even when stunting of growth is noticeable, it could be caused by other factors, including poor cultural practices, inadequate moisture or nutrient deficiency. During dry weather, the diseased cane will often show signs of drought stress earlier than healthy cane, but with adequate moisture, visual detection of differences may be difficult or impossible.

Although there may be no externally conspicuous symptoms of the disease, internally there is usually an orange-red discoloration of the vascular bundles containing the water-conducting tissues (xylem) at the basal nodes of the stalk. Similar discoloration is also associated with other sugarcane diseases so it is not a totally reliable indicator of RSD. The discoloration of RSD, however, does not extend into the internodes as it does with some other diseases. In diseased fields where stunting is apparent, the shortening of stalks is not usually uniform from stool to stool. Such fields may show an "up and down" appearance. The RSD causing organism, Clavibacter xyli subsp. xyli , is a small aerobic bacterium. The RSD bacterium is transmitted through seed cane taken from diseased plants. Because symptoms of the disease are not readily visible, the bacterium may be spread unwittingly from one area to another.

Since RSD bacteria are easily transmitted mechanically, sanitation is important in preventing healthy cane from becoming infected. All cane cutting implements should be protected from contamination from diseased cane or be disinfected before use on

healthy cane. Disinfection can be achieved by heat or chemicals. Chemical disinfectants that may be used on cane cutting knives include Lysol, Dettol, ethanol, Mirrol and Roccal. At least 5 minutes of contact with the cutting surface is needed to assure disinfection, but a shorter disinfection time will be partially helpful. Heat treatment of seed cane before planting is used to eliminate bacteria prior to the establishment. Resistant clones have been shown to be the easiest way to control the disease.

2. Red Rot of Sugarcane

Red rot occurs in various parts of the cane plant but it is usually considered a stalk and a seed-piece disease. Its symptoms are highly variable depending upon the susceptibility of the sugarcane variety and the environment. Symptoms may not be readily apparent in the field, especially in the early stages of the disease. In the later stages of the disease, red rot may cause standing cane to "break down" .  Sugarcane lodging caused by Red Rot. Diagnostic symptoms can best be observed by splitting the stalk lengthwise. The infected tissues have a dull red color interrupted by occasional whitish patches across the stalk (Figure 2). These white patches are specific to the disease and are of significance in distinguishing red rot from other stalk rots. Reddened vascular bundles may also pass through to the healthy tissues. In susceptible varieties the red color, sometimes along with some gray color, may be seen throughout the length of the stalk. The infection is largely confined to the internodes in resistant varieties. On the leaves, the pathogen may produce elongated red lesions on the midribs, reddish patches on the leaf sheaths, and, infrequently, small dark spots on the leaf blades. The lesions may eventually develop a straw color in the center. In seed pieces, the entire seed piece may become rotted and the internal tissues turn various shades of red, brown or gray.

of seed cane nurseries.

Red rot disease is caused by the fungus Glomerella tucumanensis. An older name, Colletotrichum falcatum, is still preferred by some pathologists. The red rot fungus can be readily isolated from infected tissues. At least two races have been identified.  Fungus growth is affected by temperature, pH, nutrition and environmental conditions.

Midrib lesions are probably the major source of inoculum during the growing season. Diseased stalks generate a great deal of inoculum. Dissemination of the inoculum takes place by wind, rain, heavy dews and irrigation water. Infected plant material can readily spread or cause secondary infections. Crop debris or stubble may also provide inoculum to infect a new crop. Although the fungus is not a true soil-borne organism, spores washed into the soil may produce infection in planted seed pieces. Hosts other than sugarcane are not considered important inoculum sources. Climatic factors affect both the spread and severity of red rot. In newly-planted cane, the disease is favored by excessive soil moisture, drought conditions, and low temperatures.

Resistant varieties have been the most effective method of prevention and control. The incidence of red rot can be reduced through good cultural practices, such as clearing fields of excessive trash and efficient drainage. Agronomic practices that hasten germination are important in reducing seed rotting and obtaining good stands. The avoidance of planting susceptible cultivars during excessively cool and wet weather has been effective in several countries. Regular roguing of diseased plants, burning of trash, plowing out badly affected fields, maintenance of proper soil moisture, and prompt harvesting of infected or susceptible crops are other management practices recommended for red rot control. Heat treating of seed cane has also been effective in controlling seed piece infection of red rot.

3. Sugarcane Smut

Sugarcane smut was first noted in South Africa in 1877. It is caused by the fungus Ustilago scitaminea. The disease is sometimes referred to as "culmicolous" smut of sugarcane because it affects the stalk of the cane.Sugarcane smut does not always pose a serious problem where it occurs. However, smut may remain unnoticed for years, then quickly devastate large areas of susceptible varieties. Hence, the disease has been called the "dread disease of sugarcane" by some and a "trivial disease with exaggerated yield losses" by others. Smut can cause significant tonnage losses as well as juice quality losses. Disease development is dependent on the environmental conditions and the resistance of the sugarcane varieties grown.

The most recognizable diagnostic feature of a smut infected plant is the emergence of a "smut whip." A "smut whip" is a curved, pencil-thick growth, gray to black in color, that  emerges from the top of the affected cane plant (Figure 1). These "whips" arise from the  erminal bud or from lateral shoots on infected stalks. They can vary in length from a few inches to several feet long. The whip is composed partly of host plant tissue and partly of fungus tissue. Whips begin emerging from infected cane by 2-4 months of age with peak whip growth occurring at the 6th or 7th month.

Sugarcane smut is spread by microscopic spores. The spores are particularly adapted to aerial dispersal and can be spread over great distances by wind currents. The whip serves as a source of spores. It has been shown that approximately one billion spores per whip per day can be released into the air. Standing cane becomes infected in the buds. Since many infected buds remain dormant until the cane is cut for seed and planted, the use of infected seed cane is another important way the disease is spread.

Disease free seed, rouging and hot water treatments have been found successful in prevention and control of the disease. However, the later two become ineffective as scale of the work increases.

4. Sugarcane Rust

Sugarcane rust is mainly a disease of the leaf. The earliest symptoms are small, elongated yellowish spots that are visible on both leaf surfaces. The spots increase in length, turn brown to orange-brown or red-brown in color, and develop a slight, but definite, chlorotic halo. Lesions typically range from 2-10 mm in length but  occasionally reach 30 mm. They are seldom more than 1-3 mm in width. Infections are usually most numerous toward the leaf tip, becoming less numerous toward the base.

Pustules, which produce spores, usually develop on the lower leaf surface.

Pustules may remain active over a considerable period of time and spore production is highly dependent upon climatic conditions. However, eventually lesions darken and the surrounding leaf tissues become necrotic. On a highly susceptible variety, considerable numbers of pustules may occur on a leaf, coalescing to form large, irregular, necrotic areas. High rust severities may result in premature death of even young leaves. Severe rust has caused reductions in both stalk mass and stalk numbers, thereby reducing cane tonnage.

Sugarcane rust is caused by the fungus Puccinia melanocephela. An obligate parasite, the pathogen incites new infections only on living host tissue. Changes in varietal susceptibility to rust have been observed over the years, suggesting the existence of fungal variants.

Rust spores are very well-suited to dissemination by air currents. Rust epidemics have been demonstrated to develop in the direction of prevailing winds. Leaf wetness and atmospheric temperature are the environmental factors most influential for rust development. Several hours of free moisture on the leaf surface at a favorable temperature is necessary for successful spore germination and infection, and hence, spread of the disease. While long dew periods and rainfall events both contribute to leaf wetness, rainfall events are not quite as favorable for rust development. Heavy rains tend to remove spores from the atmosphere, rendering them infective if they land on the soil.

The best means of control for sugarcane rust is to grow resistant varieties. However, resistance has not been stable or durable on certain varieties, presumably because of rust variants. For this reason, it is highly recommended that growers diversify their varietal holdings. In this way, they will not have a predominance of one variety, should a rust variant develop that is capable of infecting that particular variety. Varietal diversification may play an important role in holding down the overall area-wide disease pressure, thereby reducing the natural selection pressure for one particular rust variant. It is believed that this may assist in preserving the durability of host plant resistance in current resistant varieties.

 

5. Sugarcane Mosaic Virus (SCMV)

Mosaic is caused by a virus identified primarily by its leaf symptoms. As with most sugarcane diseases, the symptoms may vary in intensity with the cane variety, growing conditions, and the strain of the virus involved. The most distinctive symptom is a pattern of contrasting shades of green, often islands of normal green on a background of paler green or yellowish chlorotic areas on the leaf blade. Generally, the chlorotic areas are diffuse, but they may be sharply defined in some clones infected with certain strains of the virus. The infection may be accompanied by varying degrees of leaf reddening or necrosis. Chlorotic areas are most evident at the base of the leaf. Chlorotic areas may also be present on the leaf sheath, but rarely on the stalk. Young, rapidly growing plants are more susceptible to infection than more mature, slower growing plants.

There are three principal modes of spread of SCMV: (1) by aphid vectors, (2) by infected seed cane and (3) by mechanical inoculation. Only aphid vectors and infected seed cane are important in the field.

The use of resistant varieties has been found an efficient method of preventing and controlling the disease.

 

 

B. Sugarbeet Diseases

1. Cercospora Leaf Spot

Cercospora leafspot, caused by the fungus Cercospora beticola, is the most serious disease of sugarbeets. This disease can cause reduced tonnage and sucrose and increased impurities. Losses of 30 percent in recoverable sucrose are fairly common under moderate disease conditions. Roots of affected plants do not store as well in the pile as roots of healthy plants.

Cercospora infection of the sugarbeet leaf produces circular spots about 1/8 inch (occasionally 3/16 inch) in diameter with ash gray centers and dark brown to reddish purple brown borders. During warm, rainy, humid weather, the spots may coalesce and kill entire leaves, particularly on susceptible varieties. In humid weather, these coalescing spots may be covered with areas of steel blue to light bluish-purple fuzz. These are masses of spores of the Cercospora fungus, Severely diseased leaves wither and die, resulting in severe defoliation. The disease begins on the older leaves and progresses to the younger leaves. Diseased leaves usually remain attached to the crown of the plant.

Bacterial leafspot can be confused with Cercospora leafspot. The symptoms of bacterial leafspot frequently develop during cool rainy weather but may occur intemmixed with Cercospora leafspot. Bacterial leafspot often appears on leaves a week or two earlier than Cercos-pora leafspsot but can be present throughout the season. Bacterial leafspot produces irregular-shaped to circular spots that are 3/16 to 1/4 inch in diameter. They have dark gray centers Cercospora has light gray centers) with very dark to almost black borders. In areas where bacterial leaf-spots coalesce, portions of the leaf tend to tear, producing a ragged leaf.

To distinguish Cercospora leafspot from bacterial leaf-spot, examine the spots with a hand lens. The gray centers of Cercospora leafspots usually will have tiny black dots in them or if there is profuse spore production, the centers will have a fuzzy blue-gray appearance. There are no black dots or blue-gray fuzz (fungal spore mass) in the centers of bacterial leafspots.

The most common source of the Cercospora fungus is infected beet debris in the field. The fungus and spores survive over winter on this debris. The Cercospora spores are spread by wind, water (irrigation and rain), and insects. The fungus also can be carried on the seed, although this is usually of minor importance. The fungus may infect some common weeds such as redroot pigweed, lambsquarters, mallow, and bindweed, but there is little evidence that these weeds are important in the disease cycle.

Cercospora leafspot develops rapidly in warm, humid and rainy weather. The Cercospora spores are produced most readily at temperatures of 68-79 degrees Fahrenheit and relative humidities (RH) of 90-100 percent. Spores do not form at temperatures less than 50 F. Optimal spore germination and infection occur when the temperature is 75-77 F and the RH is 100 percent for at least 8 '/2 hours. In general, day temperatures of 80-90 F and night temperatures above 60 F favor disease development. Under favorable conditions, leafspot symptoms may occur in as few as five days after infection, with more spores produced in another five days.

Since the Cercospora fungus overwinters on infected beet leaves, crop rotation is important. A three-year rotation is minimal for reducing carryover of the fungus. Since plant debris and spores can be blown some distance, beets should not be planted nearer than 100 yards from a field that was in beets last year. This is especially important in cases where last year's beets were severely diseased. Burying beet refuse by tillage helps reduce inoculum survival and dispersal. Fall tillage is most effective for reducing Cercospora populations but may increase the severity of soil erosion during open winters.

Currently registered fungicides are of two types: protectant fungicides and systemic fungicides.

Protectant fungicides act on the leaf surface to prevent infection; they do not "cure" established infections. It takes five or more days from the time of infection before leafspots appear. If a protectant fungicide is applied immediately after a rainy, humid period, infections will already be established and it may be too late to prevent development of leatspots several days later. Late application may result in claims that "the fungicide didn't work" or "the fungicide wasn't applied correctly."

It is essential that protectant fungicides be on the leaf before rainy or humid weather occurs. The application must be made early enough to allow spray droplets to dry before rains begin. The fungicide is not as easily washed off after it dries.

Systemic fungicides are absorbed by the leaf. Fungicides for  Cercospora control include Benlate and Topsin M. These fungicides are related and belong to the benzimidazole class of fungicides.

2. Phoma leaf Spot and Root Rot

 Phoma betae, a fungus that attacks seedlings, leaves, and roots in the field and in storage is the causal agent of this disease. It often enters wounds during the growing season and at harvest. The disease is much worse when beets are stressed. The sexual stage of the fungus, Pleospora bjoerlingii can spread the disease within the field and to other fields by producing ascopsores while condia of the asexual stage can infect seed. Beet seed lots with 40 to 50% infection have been found.

Seedlings may damp-off. Lesions resemble those of Cercospora leaf spot. Leaves develop small brown spots with black ridges in concentric circles. Lesions become covered with small black spots (pycnidia). Roots at first develop small, depressed, brown lesions in the crown area. These become conical areas of black rot, both in the field and in storage.

Control measures include maintaining proper water and nutrient levels during the growing season and avoiding wounding roots. Seed treatments with a good fungicide.

3. Slerotium Root Rot of sugarbeet

Sclerotium root rot or southern root rot is a very destructive disease of sugarbeet. Symptoms appear as poor top growth with wilting occurring as the taproot is decayed by the fungus. Under high temperatures, plants will eventually wilt permanently. The pathogen is characterized by cottony mycelial growth on the surface of the tap root with small (1-3 mm) spherical that are tan to dark tan when mature.

Sclerotium rolfsii is a soilborne fungus that survives in the soil as sclerotia, and has a host range of over 200 plant species. The disease is favored by moist soil conditions and high temperatures, 77° to 95°F. The fungus is spread through irrigation water and by cultivation equipment. Although the disease has been reported to occur in seedlings, temperatures are not generally conducive to disease development until later in the season. Frequently, S. rolfsii can cause significant disease losses that may occur just prior to harvest, late August to early September.

There are no chemical control methods for managing this disease. Management can be best achieved by reducing inoculum buildup through crop rotation. Suggested crops to include in a rotation are alfalfa, wheat, barley, corn, or susceptible crops that do not require irrigation during warm weather conditions. Do not rotate beets with beans or other highly susceptible crops and avoid frequent irrigations during hot weather. Yield losses can be reduced through application of nitrogenous fertilizers that promote vigorous growth. Additionally, in fields where Sclerotium root rot has been identified, harvest early.

4. Powdery Mildew of Sugarbeet

Powdery mildew first shows up on the lower leaves. The first symptoms are a wispy growth of white to light gray threadlike filaments, often radiating from a central point. Early symptoms of powdery mildew are not detected easily. Early symptoms are most readily detected under full sunlight, with the sun to your back. Once the early symptoms of mildew show up, the disease may progress very rapidly in favorable weather. A powdery white or gray-white growth may cover a leaf within a week, and the mildew may begin to show up on the middle leaves. Mildew may even show up on the upper leaves that have not completely expanded when disease is severe. A characteristic odor similar to that of a musty basement may be noted in fields with severe mildew.

Powdery mildew is less severe where nitrogen fertility is high. When inspecting a field for powdery mildew, look in the areas of the field that are beginning to turn a light green or yellow green due to depletion of available nitrogen. Powdery mildew is not likely to be found in the rest of the field if it cannot be found in the yellowing areas. Severely mildewed leaves may begin to turn yellow within a month of initial symptoms.

Sulfur is a relatively inexpensive and effective control for powdery mildew. Some studies have also shown good results with wettable powders or flowable formulations.