Circular Circ-1025

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

Some Diseases of Vegetable and AgronomicCrops Caused by Fusarium in FloridaTom Kucharek, John Paul Jones, Don Hopkins and Jim Strandberg, respectively,Professor - Extension Plant Pathologist, Plant Pathology Dept., Gainesville; Re-tired Professor of Plant Pathology, Gulf Coast Research and Education Center -Bradenton; Professor of Plant Pathology, Mid Florida Research and EducationCenter-Apopka, Fl; and Professor of Plant Pathology, Mid Florida Research andEducation Center- Apopka, Fl.April 1992 Copied with minor revisions March2000.

Introduction

The Genus Fusarium contains many speciesof fungi that are commonly found in soil. Someare capable of causing wilts, crown rots, rootrots, or fruit rots. Others are opportunists be-cause they colonize plant tissues after sometype of stress debilitates the plant. Interestingly,many species of Fusarium that abound in thesoil are not capable of causing disease.

Fusarium wilts are caused by the speciesFusarium oxysporum. Infection of plants by otherspecies or other organisms can cause wilt, butdiseases called Fusarium wilts are caused onlyby F. oxysporum. Within F. oxysporum, specific-ity exists because some forms (form species)within the species are capable of infecting cer-tain plant species. For example, F. oxysporum f.sp. lycopersici causes Fusarium wilt in tomatoand F. oxysporum f. sp. niveum causes Fusariumwilt in watermelon. Further complicating thissituation is the existence of pathogenic raceswithin form species. One pathogenic race maycause disease in certain varieties of a crop spe-cies but not in other varieties. Other varietiesmay be infected by other pathogenic races.Form species and races of Fusarium oxysporumare indistinguishable in appearance. For theremainder of this publication, the generic term

Fusarium wilt will be used to identify the com-plex group of wilt diseases caused by F.oxysporum and its form species and races.

Other species of Fusarium cause plant dis-eases. Fusarium solani causes root rots, stem rots,crown rots and fruit rots. Fusarium subglutinansinfects sugarcane, corn, sorghum, and somebroad leaf plants. Identification of the speciesof Fusarium associated with a problem is oflittle benefit for the grower unless specific con-trol practices (e.g., resistant varieties) are avail-able for control of specific species, form spe-cies, or pathogenic races.

Some Fusarium spp., including F. oxysporumand F. solani, produce thick-walled sporescalled chlamydospores. F. subglutinans does notproduce chlamydospores. Chlamydospores arecapable of surviving in soils, soil debris, orother substrates for more than 10 or 20 years.Chlamydospores that survive in the soil ger-minate and form tube-like structures (hyphae)that penetrate roots or other plant parts. Thesespores can be carried in or on seed, transplants,wooden field stakes, clonal propagation stock,such as potato seed piece tubers, and soil at-tached to equipment.

Fusarium spp. can also survive by coloniz-ing roots or stems of so-called non-host plants.

For example, roots of barley and nutsedge canserve as colonization sites for the Fusarium wiltpathogen of cotton. Brazilian pepper, cudweed,and carpet weed will continually supportpopulations of the fungus that causes crown rotin tomato. The fungus that causes Fusarium wiltin tomato has been found to infect eggplant,mallow, Amaranthus sp., crabgrass, andricegrass. The watermelon wilt pathogen hasbeen associated with roots of citron, bitterapple, rice, peanut, and tomato. Thus, the pro-duction of thick-walled spores and the abilityof Fusarium spp. to colonize alternate plant hostsprovide mechanisms for long-term survival insoil.

Wind-blown dispersal of spores of Fusariumspp. may occur. For example, air-borne spores(conidia) of the crown rot pathogen of tomatocan recontaminate nearby fumigated sites in thefield or the greenhouse. However, for most dis-eases caused by Fusarium spp., soilbornechlamydospores are generally regarded as theprimary source of inocula.

Stress factors such as drought, flooding,chemical damage, mechanical damage, low soilpH, deposition of soil on stems while cultivat-ing, or incorrect usage of fertilizer may predis-pose plants to infection by Fusarium spp. Otherpests such as fungi, bacteria, nematodes, or in-sects may occur simultaneously in roots orstems or predispose plants to infection by theFusarium sp. For example, nematode damageaccentuates Fusarium wilt in cotton but not inany notable way in tomato, watermelon, orsweet potato. Also, lower stems of soybeansand beans are commonly infected with Fusariumsp. and other fungi after hot soil is depositedon stems during cultivation.

Root and stem diseases caused by Fusariumsp. debilitate plants by rotting (plant cell de-struction) root and stem tissue or by pluggingthe water conducting vessels (xylem) in rootsor stems. Fusarium wilts can be diagnosed ten-tatively by slicing the stem in half lengthwise.

The presence of two dark bands of water-con-ducting tissues indicates that the xylem vesselsare plugged from infection. If the stem is slicedperpendicular to the length of the stem, thedarkened tissue will often appear as a circle. Ifthe infection is one-sided in the plant, a por-tion of the circular tissue will be darkened. Theblockage of the water and nutrient-conductingtissue of the plant results in stunting, discol-oration of leaves (usually yellow, initially), andwilting. Yield losses are greater and plants aremore likely to die if infection occurs whileplants are young. Because other pests and dis-eases can cause similar symptoms, you are ad-vised to attain formal diagnoses until you be-come familiar with symptoms and field diag-nostic procedures for the crops you produce.

FUSARIUM WILTS

Fusarium Wilt of Tomato

Fusarium, wilt of tomato (Fig. 1) would bemore common and devastating throughoutFlorida if resistant varieties were not used. Re-sistance to Fusarium wilt in tomato is complete;that is, all plants within a variety are resistantto the designated races unless a seed or trans-plant mishap has occurred. Currently, races 1,2, and 3 exist in Florida. If Fusarium wilt ispresent, slicing the stem lengthwise into equalhalves will reveal two dark, orange-red, orbrown streaks of darkened water-conductingvessels (Fig. 2). Symptoms include lower leafyellowing and then browning, stunting, vascu-lar discoloration, and plant death. Leaf yellow-ing is often seen on one side of the plant andfrequently in leaflets on one side of the leaf. Thedarkened vascular tissue often extends up tothe middle part of the stem and sometimes tothe top of the stem and into the vascular tissueof the fruit.

The optimum temperature for developmentof Fusarium wilt and growth of the fungus is

82°F. Growth of the fungus is reduced at tem-peratures below 68°F or above 93°F. However,Fusarium wilt has occurred in Florida at tem-peratures above 95°F.

The primary control for Fusarium wilt of tomatois the combined use of crop rotation, resistantvarieties for the pathogenic races in your area,disease free transplants, and preplant fumiga-tion with a broad spectrum fumigant (e.g., me-thyl bromide + chloropicrin, vapam, etc.).Reinfestation of fumigated soil with non-fumi-gated soil and pond water should be avoided.Transplant production areas should be sanitaryand should not be adjacent to field productionareas or equipment cleaning areas.

Sanitizing used field stakes will reduce, notnecessarily eliminate, inoculum associated withthe stakes that can infest fumigated soil. Whentomatoes are planted in a new field, it is best touse new stakes because sterilization proceduressuch as steam or chemicals may not eliminateall of the inoculum. When planting tomatoesin a field that contained tomatoes in the past,use of sterilized stakes is preferred. However,with use of unsterilized stakes, the crop maybe near the end of harvest before the wilt fun-gus causes any noticeable problem.

Additional controls for Fusarium wilt of to-mato include adjusting pH, fertility and mois-ture of the soil. The soil pH should be 6.5 orabove coupled with minimum usage of ammo-nium- containing fertilizer. Avoid the use ofexcessive amounts of nitrogen, phosphorus,and magnesium. Soil moisture should be ad-equate for crop production but not excessive.Use of drip or seepage irrigation coupled withadequate drainage for heavy rains will providea proper soil moisture situation. These adjust-ments to the soil will be most effective for con-trol of Fusarium wilt if the entire root system isconfined within the treated soil such as thatwithin a plastic mulch system.

This fungus can be seedborne. However this

source of inoculum is not common. The impor-tance of seedborne inoculum, relates to the po-tential for introducing new races from other ar-eas of the world.

Fusarium Crown Rot of Tomatoes

Fusarium crown rot of tomatoes (Fig. 3) hasbecome widespread in southern Florida on thesandy, acidic soils and it has been found occa-sionally on the high pH soils in Dade County.Symptoms of Fusarium crown rot are distinctlydifferent from those of Fusarium wilt of tomato.The brilliant yellowing of the foliage typical ofFusarium wilt does not occur with crown rot.With crown rot, the leaflets often have a mar-ginal necrosis (death). Infected plants often wiltduring the day and recover during the night(Fig. 4). With crown rot, definite root and crownrots occur (Fig. 5). Additionally, unlikeFusarium wilt, the vascular discoloration is lim-ited to the lower 12" of the stem (Fig. 6). An in-fected plant rarely dies but infected plants willbe stunted. Even with severe vine symptoms,some yield is attainable. This fungus is calledF. oxysporum f.sp. radicis-lycopersici. It grows bestfrom 50°F to 68°F which is lower than the opti-mum for the fungus that causes Fusarium wilt.The optimum temperature for disease devel-opment is 70°F .

Control of crown rot is similar to that ofFusarium wilt (e.g., crop rotation, sanitation,increase soil pH, minimize use of ammoniacalnitrogen, and soil fumigation). Resistant vari-eties for control of crown rot are currently avail-able only for greenhouse production. Crown rotis likely to occur with a higher frequency wheredirect seeding is used, instead of healthy trans-plants, and where the soil contains high levelsof chloride salts. The utmost sanitation produc-tion scheme for transplants in greenhousesshould be used so that individuals or equip-ment used within or around the transplant sitedo not become contaminated with disease-caus-ing organisms from the field. Considerable in-formation on sanitation within transplant sys-

tems is available in Plant Protection Pointer No.25. Finally, transplants should be transported,pulled, and set without tissue damage as dam-aged tissues are likely to be sites for infection.

Fusarium Wilt of Watermelon

Fusarium wilt of watermelon is common inFlorida where resistant varieties are not usedbut it occurs to some extent even when resis-tant varieties are used. Resistance to Fusariumwilt in watermelon varieties is not complete;that is, some plants within the “resistant vari-ety” may be susceptible. However, a suscep-tible variety contains a higher percentage ofsusceptible plants than a resistant variety. Threeraces (0, 1 & 2) exist and resistance to all threeis necessary. Currently, resistance to race 2 hasnot yet been incorporated into a commercialvariety.

Fusarium wilt of watermelon usually occurswithout plant yellowing; usually the entireplant wilts quickly (Fig. 7), becomes brown anddies. Occasionally, wilting of vines on one sideof the plant occurs, particularly on older plants.Slicing the tap root lengthwise into two equalhalves will reveal two streaks of vascular tis-sue that are dark yellow-brown, orange brownor reddish brown (Fig. 8). In Florida, Fusariumwilt is likely to occur prior to fruit set. The op-timum temperature for infection is near 80°Fwith little infection occurring above 86°F. Thiswilt pathogen has been found to be associatedwith seed but the importance here would bethe potential of introducing a new race into thefield from other areas of the world.

The best control for Fusarium wilt of water-melons is the use of resistant varieties coupledwith crop rotation. Where land is limiting foran adequate rotation scheme on your farm, twoalternatives exist. First, you may lease land thathas not had watermelons for many years, Sec-ondly, the use of certain varieties (e.g., Crim-son Sweet) on the same land year after year, al-though not advised, may result in less

Fusarium wilt than if other varieties are planted.Rotation includes the absence of watermelonfor five years or more. Also, some varieties ofyellow summer, zucchini, and scallop squashare susceptible to this fungus and should beexcluded from the rotation.

Additional controls include the use of certi-fied, disease-free transplants and delayed thin-ning of direct seeded stands. Delayed thinningpermits a better chance of culling susceptibleplants rather than resistant plants within thepopulation. The use of higher soil pH’s and lessammoniacal nitrogen has not consistently re-duced the amount of wilt in watermelons as ithas in tomatoes.

Fusarium Wilt of Cantaloupe

Fusarium wilt of cantaloupe has not beenformally identified in Florida. However, it islikely that it has occurred in the panhandle re-gion of Florida. Symptoms are similar to otherFusarium wilts. This wilt pathogen has beenfound on seed. This disease is favored by cooltemperatures between 65 to 77°F .

Fusarium Wilt of Cucumber

Fusarium. wilt of cucumber occurs occasion-ally in Florida, usually where cucumbers aregrown on the same land year after year. Symp-toms are similar to that of Fusarium wilt ofwatermelon and cantaloupe. Controls includecrop rotation, and liming of soil coupled withuse of fertilizer with a minimum amount ofammonium salts.

Fusarium Wilt of Cotton

Fusarium wilt of cotton occurs infrequentlyin Florida. Prior to the use of varieties with re-sistance, this disease was more prevalent inFlorida. Infection of cotton by the Fusarium wiltpathogen is highly dependent upon damage

from nematodes but does not necessarily oc-cur when plants are infected with nematodes.Symptoms include stunting, lower leaf yellow-ing and browning, wilting (often seen first atflowering), gradual reduction of plant vigor,and possibly plant death. Vascular discolora-tion may be dark brown to black. Controls in-clude crop rotation, suppression of nematodesand use of varieties resistant to prevalent races.

Fusarium Wilt of Tobacco

Fusarium wilt of tobacco is present occasion-ally in Florida. Symptoms include leaf yellow-ing and wilting (Fig. 9). Leaf yellowing andwilting may occur on one side of the plant butnot the other, a symptom not uncommon withFusarium wilts. Injury from cultivation ornematodes will predispose plants to infection.Calcium deficiency will accentuate Fusariumwilt of tobacco. Fusarium wilt of tobacco willoccur across a wide range of soil pH’s, but it islikely to be more severe at 7.0. This wilt funguscan grow from 45 to 95°F but grows best be-tween 77 to 86°F. This disease is best controlledby crop rotation, resistant varieties and reduc-tion of nematode damage. Sweet potato shouldnot be used as a rotational crop as they are alsosusceptible to this fungus.

Fusarium Wilt of Sweet Potato

Fusarium wilt of sweet potato occurs occa-sionally in Florida if resistant varieties are notused. Leaf yellowing and browning of the old-est leaves during vine elongation is a commonsymptom. Stunting and eventually plant deathmay occur. Vascular discoloration may be simi-lar to that of other wilts with two discoloredbands being evident when the lower stem aboveor below the soil surface is cut lengthwise.However, if the wilt is one-sided, the vasculardiscoloration may be only on one side of thestem. Vascular discoloration will vary in colorfrom brown to purple. Infection can occur any-time during the growth of the crop but infec-

tion is most likely during or shortly after trans-planting. Damage to roots from pulling trans-plants or any other factor favors infection. Thisfungus grows at all temperatures that are fa-vorable for crop production but may be inhib-ited to some degree at soil temperatures above86 to 95°F. Control includes crop rotation, re-sistant varieties, selection of healthy trans-plants, minimizing stress and avoidance ofplanting when soil is cool.

Fusarium Wilt of Crucifers (Cole Crops)

Fusarium wilt of crucifers has occurred spo-radically in Florida in cabbage, collards andradish but generally this disease has not im-pacted crop production significantly. Other cru-ciferous species are also susceptible. Becauselittle is known about the degree of susceptibil-ity of different cruciferous varieties, except forcabbage, the occurrence of this disease in othercruciferous crops may occur in the future.Symptoms include seedling death (Fig. 10),stunting, stem curling, leaf drying on the edges,yellowing of lower leaves, dropping of leaves,bud formation on leafless stems, vascular dis-coloration, and often plant death. Black rot, abacterial disease, can be confused withFusarium wilt because it causes black veins instems, roots and leaves. Fusarium wilt is mostlikely to be a problem in plantings that are ini-tiated in the late summer to early fall or mid tolate spring. The fungus grows best at 80 to 90°Fand is strongly inhibited below 61°F and above95°F .

Control of Fusarium wilt in crucifers in-cludes use of crop rotation and disease-freetransplants. For cabbage, many resistant vari-eties are available. When cabbage or other cru-cifers are grown without crop rotation, use ofresistant varieties is essential.

Fusarium Wilt of Soybeans

Fusarium wilt of soybeans has been foundon occasion in Florida but the extent of its inci-

dence and importance is unknown. Symptomsinclude lower leaf yellowing, leaf drop, stunt-ing, wilting in mid season, and possibly plantdeath. Infection is enhanced by cool tempera-ture from 57°F to 74°F . Thus, this disease mightbe a problem if soybeans are planted too early(prior to mid May) or if soils remain cool andwet into the normal planting season. Wound-ing from cultivation, nematodes and herbicidesenhance the development of this disease. Re-sistance exists, but the extent of use of this re-sistance by plant breeders is not known.

Fusarium Wilts of Beans and Chickpeas

Fusarium wilts of beans and chickpeas haveprobably occurred in Florida based upon symp-toms (Figs. 11 & 12) but official documentationis not available.

Fusarium Root and Lower Stem Rots

Fusarium spp. are commonly associated withroots and lower stems of unthrifty plants. Asindicated earlier, the presence of Fusarium spp.in roots or lower stems does not indicate thatthe Fusarium spp. are the cause of the problem.However, primary or secondary invasion ofplant tissue by Fusarium sp. can cause root de-bilitation which is usually followed by stunt-ing, loss of green color in leaves, wilting, andother decline symptoms. Some crops that arecommonly infected with Fusarium spp. inFlorida include celery (red root, Fig. 13), pars-ley (Fig. 14), carrot, corn (particularly field corn),sorghum, millet, peanut (Fig. 15), soybean,snow peas (possibly a wilt type of Fusarium),beans, southern peas, forage legumes, onions,and some herbs. Probably, most plants sustainsome damage from Fusarium spp.. Infection byFusarum sp. often begins during the seedlingstage of the plant for direct-seeded crops (Fig.16). Fusarium-induced diseases of potatoes(Fig.17) would be more of a problem in Floridaif certified seed pieces were not used.

Cultural and chemical control practices pre-

sented for Fusarium wilt diseases will reduceFusarium root rots. Also, recontamination offumigated soil should be avoided because fu-migated soil has less natural biological diver-sity. Soils with less microbiological diversitymay allow for a rapid increase of a plant patho-gen (Fig. 14). Fertility adjustments may not beas effective for Fusarium root rots as they arefor Fusarium. wilts. Additional controls includefungicide seed treatments, destruction of allgreen manure 30 days or more prior to plant-ing, preparation of soil into a loose tilth forplanting, avoidance of deep setting of seeds ortransplants, use of healthy, undamaged trans-plants, and care not to deposit soil on stemswhen cultivating.

Fusarium Fruit and Flower Rots

Infection of flower or fruit parts by Fusariumspp. has not been common in Florida but bothoccur. Peanut and corn are the crops that aremost likely to incur infection of fruit by Fusariumspp (Figs. 18 & 19). Infection of peanut podscan be associated with damage from soil insectsand nematodes. Fruit (ears) of field corn arecommonly infected with Fusarium spp., particu-larly if damage to the husk or silks occurs fromhail, insects, etc. Some Fusarium spp. that infectgrain crops such as corn and wheat producetoxins that can poison livestock.

Occasionally, fruit of vegetables may be in-fected with Fusarium spp. (Fig. 20). Control forsuch fruit rots include avoidance of damage tofruit from insects or other factors, trellisingcrops or use of plastic mulch so fruit are notproduced in direct contact with the soil. Theinfection of flowers by Fusarium spp. is not acommon problem but has been noted in tomatoflowers infested with thrips, a small insect.

Summary

It is not possible to adequately generalizeabout the complex range of symptoms, envi-ronmental factors, and controls associated with

Fusarium-induced diseases for the many sus-ceptible crops. The reader is advised to makeevery effort to seek professional advice on theimportance of the Fusarium spp found in asso-ciation with a given situation. However, numer-ous cultural and chemical controls can be rou-tinely incorporated during the production of acrop so that the impact from Fusarium spp. will

be minimized. As noted throughout this publi-cation, successful control of Fusarium-induceddiseases begins with healthy seed, healthy seedpieces, healthy transplants, and properly pre-pared and rotated land. Changes of soil fertil-ity, use of resistant varieties, avoidance of plantstresses (biological and physical), and culturalmanipulations can be used successfully in somesituations.

Figure 1. Wilting of tomato plants withFusarium wilt.

Figure 2. Tomato stem with discol-ored xylem tissue from Fusariumwilt.

Figure 3. Fusarium crown rot of tomato. Figure 4. Wilting of tomatoplants with crown rot.

Figure 5. Lower stem of tomato with crownrot.

Figure 6. Inner tomato stem with crown rot.

Figure 7. Wilting of watermelon plant withFusarium wilt.

Figure 8. Inner watermelon tap rootwith Fusarium wilt.

Figure 9. Fusarium wilt of tobacco. Figure 10. Fusarium wilt of cabbage seed-lings.

Figure 11. Fusarium wilt-like symptoms inbeans.

Figure 12. Fusarium wilt-like symptoms inchickpeas.

Figure 13. Fusarium root rot (red root) incelery.

Figure 14. Seedling blight of parsley causedby Fusarium.

Figure 15. Fusarium-infected peanuts. Figure 16. Onion seedlings infected withFusarium.

Figure 17. Fusarium-infected potato tubers.

Figure 18. Peanut pods with Fusarium andother fungi.

Figure 19. Fusarium ear rot of field corn.Photo courtesy of Dr. Johnny Crawford,University of Georgia.

Figure 20. Pumpkin fruit rot caused byFusarium.