Plant Pathology Fact Sheet PP-41

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Rhizoctonia Diseases in Aboveground PlantParts of Agronomic and Vegetable CropsTom Kucharek, Professor and Extension Plant Pathologist in the Plant Pathol-ogy Department, University of Florida, Gainesville, FL 32611. May 1990; CopiedFebruary 2000.

Introduction

Rhizoctonia solani is a soilborne fungusthat is common in Florida. It is capable of exist-ing and growing in the soil in the absence orpresence of plants. The host range of R. solani isextensive. Most crops are susceptible to the fun-gus as a seedling blight, root rot, fruit rot oraboveground aerial blight. R. solani commonlycauses seedling blights and root rots in plantsand is therefore often regarded as a belowground pathogen only. That perception is in-correct. This fungus consists of different“groups” within the species which provide fora diversity of pathogenic potential. In warm andhumid climates, such as in Florida, R. solanicauses many plant diseases in abovegroundplant parts in addition to those caused inbelowground plant parts. This publication pre-sents information about diagnosis and controlof plant diseases caused by Rhizoctonia in plantparts above the soil surface. Plant PathologyFact Sheet 1 discusses this same fungus in rela-tion to seedling blights.

The primary habitat of Rhizoctonia is thesoil. When conditions are suitable, hyphae ofRhizoctonia may penetrate plant tissues belowthe soil surface. The fungus grows as smallthreads (hyphae) in soil. It is abundant in andon dead and living plant litter, which is on ornear the soil surface. Also, sclerotia (small pin

cushion-like masses of hyphae) may be formedby the fungus and allow for long-term survivalin soil or water (Fig. 6). As decomposition ofplant litter occurs, beneficial fungi and bacte-ria colonize such litter, and Rhizoctonia is re-duced.

In some situations, the fungus may causediseases in plant parts above the ground.Rhizoctonia is often characterized as a non-spore-forming fungus, but under certain con-ditions with some isolates of the fungus, ba-sidiospores are ejected above the soil surfaceor hyphae grow upward on the plant. Wheneither of these situations occur during warmtemperatures with high humidities, infectionstructures are formed, foliar plant tissues arepenetrated, and disease occurs.

Symptoms

Rhizoctonia causes different types ofsymptoms in plant parts above the soil surface.These symptoms may be classified into four cat-egories: leaf spots, blights, lower stem rots, andfruit rots. These diseases are commonlymisidentified in the field because of the mis-conception that Rhizoctonia is limited to theinfection of plant parts in the soil. Also, Rhizoc-tonia-induced diseases on aboveground plantparts sometimes appear similar to other dis-eases.

Leaf spots may be 2 inches or more indiameter or less than 1/4 inch in diameter. Allspots begin from a single point of infection andtherefore the earliest sign of a leaf spot may beno more than a pinpoint-sized dark spot (Fig.1). After infection, the fungus grows on and inthe leaf and causes small spots (Fig. 1) or largerspots that have concentric rings with alternatelight or dark areas (Figs. 2, 3, and 4). Colorationof spots varies from a light brown to dark brown(Figs. 1, 2, 3, and 4). In some situations, the cen-ter of a spot will dry, become brittle, and sepa-rate from the leaf causing shot holes (Fig. 1) orlarge torn areas (Fig. 3). The target-shaped spotsin tobacco leaves (Figs. 2 and 3) are similar inappearence to spots caused by Alternaria tenuis(brown spot). Microscopic diagnoses are rec-ommended for such situations to determinewhich organism is causing the disease.

Blights caused by Rhizoctonia are clas-sified differently from spots because they tendto be larger in size and are not necessarilyround. Blights caused by Rhizoctonia are com-monly called aerial blights. Diseased tissue isirregular in shape. Blighted leaves have lightbrown spots that are lobed (Figs. 5, 7, and 8).Tissue that has been recently infected may begreasy in appearance due to the breakdown ofcells in the tissue (Figs. 4, 5, and 6). Some of thepeanut leaflets in Fig. 4 have leaf spots andblights. Early in the morning or shortly after awetting event, hyphae (strands) of the funguscan be seen on or between tissue (Fig. 6). Thesestrands are sometimes confused with the web-bing produced by spider mites. Sometimesaerial blights caused by Rhizoctonia are re-ferred to as web blights.

Rots of plant parts just above the soilsurface may be an extension of disease just be-low the soil surface (Figs. 9, 10 and 11). Rottedareas begin as small reddish-brown orange dis-colorations on the tissue and later the funguscauses sunken or cratered rots (Figs. 9 and 10).When such infections occur, plants may havediscolored leaves and upper plant parts due to

the reduction of upward translocation of nutri-ents and water from the soil. Later plants mayfall over (lodging) or die. With crops such ascelery (Fig. 9) and onions (Fig. 10), the edibleportion of the plant becomes unusable. Withsalad crops such as lettuce, endive, and spin-ach, which naturally have leaves that touch thesoil, a rot occurs on the stem and lower leaves(often called bottom rot). Such rots may not benoticed until the plants wilt, die, or are har-vested. Also, secondary soft rot bacteria oftenfollow such infections; this results in mushy tis-sue.

Brace roots of field corn which supportthe plants may be rotted which results in lodg-ing and lower yields (Fig. 11). A disease calledlimb rot occurs in peanut vines that are in con-tact with the soil. Lesions of limb rot may betwo inches or more in length, light brown incolor, and may contain concentric rings. Pegsof peanuts may also have lesions caused byRhizoctonia. These lesions vary in color, andmay be infected with other fungi also. Labora-tory tests are essential for such diagnoses. Rotsof plant parts at or just above the soil surfacesometimes originate during the seedling stageor on transplants.

Fruit rots caused by Rhizoctonia occureither because the fruit is in direct contact withthe soil or is part of an aerial blight infection.Symptoms in fruits are variable when infectedwith Rhizoctonia, but they usually occur on theside of the fruit that contacts the soil. In cucum-bers, lesions begin as slightly raised blister-like(edema) spots that are dry or greasy. Later, thesespots enlarge, become reddish-brown-orangeand sunken (Fig. 12). This disease is called bellyrot and has caused culling or more than 50% ofthe fruit. A similar disease occurs in wintersquash (Fig. 13) and pumpkin (Fig. 14) fruitwhere the fruit is in contact with the soil. Withwinter squash, the infected area often has raisedbumps (edema) or netting (Figs. 13 and 14).When fleshy fruits, such as tomatoes or beans,are in contact with the soil, Rhizoctonia causes

a disease called soil rot (Figs. 15 and 16). WhereRhizoctonia causes an aerial blight, the fruitbecomes greasy (Fig. 8) and seed is reduced insize, rotted, or not produced.

Sometimes the fungus, Pythium sp.causes fruit rots that initially appear similar torots caused by Rhizoctonia. As the diseaseprogresses, however, Pythium-induced dis-eases often form large masses of pure whitehyphae (mycelia) (Fig. 17). Cucumber fruit maybe infected with both fungi.

Control

Control of Rhizoctonia-induced diseasesin aboveground plant parts begins prior to theplanting of the crop. Avoid planting in low andwet, or poorly drained land. The soil shouldbe plowed with a mold board plow to bury oldplant debris which reduces the amount ofRhizoctonia near the soil surface where it sur-vives best. Soil preparation should be com-pleted 30 days prior to planting so that the ben-eficial soil microflora have ample time to rotold debris and become established as naturalbiological controls against Rhizoctonia. Neverplant a crop into freshly tilled soil where greenplant matter has not decomposed. Diseasescaused by Rhizoctonia are commonly more se-vere in a crop which follows legume crops suchas beans, southern peas, or soybeans. Thereforecrop rotation that excludes legumes should bebeneficial.

Other cultural controls include theavoidance of deep seeding, deep transplanting,dense planting schemes, excessive overhead ir-rigation, and excessive fertilization (especiallynitrogen) that promotes rank foliar growth. Re-member, aerial blights caused by Rhizoctoniaare more likely to be a problem where exces-sive humidities exist during warm weather.Crops should be planted when soil tempera-tures are ideal for rapid seed germination orplant growth. This reduces the amount of in-

oculum in young plants that can serve as asource of disease later in the season. Purchasehealthy transplants or propagation stock froma reputable transplant producer.

Fruit rots of vegetables caused byRhizoctonia and certain other fungi that origi-nate in the soil can be reduced significantly bytrellising the crop or by using a full-bed mulch(e.g., plastic) on which the fruit can form. Anymethod that reduces direct contact of soil withfruit would serve the same purpose.

Chemical controls include seed treat-ment with an effective fungicide, soil fumiga-tion with a broad spectrum liquid or gaseousfumigant, and sometimes a fungicide spray.Seed treatments reduce the amount of early sea-son inoculum. Likewise, soil fumigation re-duces the initial fungal population. Where soilfumigation is used, it is important to avoid re-contamination of fumigated soil by not mov-ing soil from unfurnigated areas into the fumi-gated area. Contamination of fumigated soil iscommonly done with tillage equipment, soil onfootware, and flowing water from excessive rainor irrigation, Aerial blights and sometimes fruitrots caused by Rhizoctonia can be reduced byfungicide sprays. Contact your County Exten-sion Office for current information.

Figure 1. Tobacco leaf with shotholes.

Figure 2. Tobacco leaves with round brownspots.

Figure 3. Tobacco leaves with large spots andblight-type lesions.

Figure 4. Peanut leaflet with spots andblight.

Figure 5. Peanut leaves with aerial blight.

Figure 6. Aerial blight in peanuts. Figure 7. Southern pea leaflets with aerialblight.

Figure 8. Aerial blight in soybeans. Figure 9. Stalk rot in celery.

Figure 10. Bulb and leaf rot in onions. Figure 11. Brace root rot in field corn.

Figure 12. Belly rot in cucumber. Figure 13. Belly rot in spaghetti squash.

Figure 14. Belly rot in pumpkin. Figure 15. Soil rot in tomato.

Figure 16. Soil rot in beans. Figure 17. Cottony leak in cucumber causedby Pythium spp.