128

Proc. Fla. State Hort. Soc.

115: 2002.

Proc. Fla. State Hort. Soc

. 115:128-130. 2002.

WEB BLIGHT OF EAST INDIA HOLLY FERN CAUSED BY

RHIZOCTONIA

SOLANI

T

ILLMAN

F. W

OOD

, IV, R

OBERT

T. M

C

M

ILLAN

, J

R

.

1

AND

W

ILLIAM

R. G

RAVES

University of FloridaTropical Research and Education Center

18905 SW 280 StreetHomestead, FL 33031

Additional index words. Arachniodes

simplicor

var. “

Variegata

”,

Thanatephorus

cucumeris

, Koch’s postulates

Abstract.

In December 1999, in a south Florida commercialnursery, 10% of the

Arachniodes simplicor

“variegata” (Mak.)Ohwi ferns in containers showed necrotic areas on the fronds.

Rhizoctonia solani

Kuehn was isolated from the ferns on cornmeal agar. Five 3

×

3 mm pieces of uninoculated potato-dex-trose agar were placed on eight control plants sprayed with tapwater. The plants were then placed in plastic bags and allowedto incubate for two days at room temperature. Eight treatedplants were sprayed with tap water. These plants were then in-oculated with five 3

×

3 mm pieces of potato-dextrose agarfrom a 3-day-old

R. solani

culture and placed in the plasticbags and allowed to incubate at room temperature (25

°°°°

C) fortwo days. The bags were removed and plants were placed in agreenhouse (20

°°°°

C night/28

°°°°

C day) and observed for signs ofinfection. Infection appeared as necrotic areas around the in-oculum within two days. The infection progressed along thefrond as a spreading light brown necrosis that eventually en-compassed most of the frond. The fronds changed color fromtheir normal dark green with yellow to light green center varie-gation and then to brown within one month. The fungus con-sistently reisolated from the plants was

R. solani

.

This research was supported by the Florida Agricultural Experiment Sta-tion, and approved for publication as Journal Series No. N-02235.

1

Corresponding author.

The fungus

Thanatephorus cucumeris

(Frank) Donk with

Rhizoctonia solani

Kuehn as its anamorph was first reported in1918 (Chupp and Sherf, 1960). Since then it has been ob-served in the U.S. from the Gulf States to as far north as Vir-ginia, and worldwide in Brazil, Burma, India, Ceylon, Japan,and the Philippines (Chupp and Sherf, 1960). The fungus

Fig. 1. Rhizoctonia mycelial growth in culture.

Fig. 2. Inoculated plug resting on fern leaflet.

Fig. 3. Uninoculated plug resting on fern leaflet.

Proc. Fla. State Hort. Soc.

115: 2002. 129

attacks some 100 different types of plants, including both cul-tivated ones and weeds (Alfieri et al., 1991; Dwivedi andDubey, 1987; Pirone, 1970; Preston, 1968; Sharma and Sanka-ran, 1984). Currently, south Florida has approximately 400acres devoted to the production of flowering annual land-scape plants. In south Florida,

T. cucumeris

occurs on plantleaves, stems and pods during the wet warm summer months.Over the past years

T

.

cucumeris

was found on the stems andleaves of commercial nursery potted

Sophora tomentosa

L.,commonly known as silverbush (McMillan et al., 1994),

Cu-paniopsis anacardioides

(A. Rich.) Radlk., carrot-wood, (Mc-Millan et al., 1994),

Breynia distichia

L., and ‘Dwarf HawaiianSnowbush’ (McMillan et al., 1997).

In south Florida, the fern

A. simplicor

is grown in contain-ers, both in gardens and also in the florist trade. An outbreakof a foliar disease on

A. simplicor

was first observed in Decem-ber of 1999 in a large commercial nursery in Miami-DadeCounty, Florida.

Rhizoctonia solani

was positively identified asthe cause of web blight on the fern with 10% of the grower’scrop infected, showing extensive necrotic areas on the fronds

rendering the plants unmarketable. This pathogen presents aserious and continuing problem to growers. The purpose ofthis study was to reproduce the disease in

A. simplicor

and testKoch’s postulates.

Methods and Materials

A culture of

T. cucumeris

(Fig. 1) originally isolated fromnaturally infected

A. simplicor

on corn meal agar with 100 mgof chloramphenicol per liter was transferred periodically on

½

strength potato dextrose agar and maintained at 22

°

C.This isolate was employed throughout this study. Four plantswere misted with water and randomly inoculated with 3

×

3mm agar blocks containing the isolate (Fig. 2). Four controlplants were misted with water and received agar blocks with-out the pathogen (Fig. 3). All plants were placed in sealedpolyethylene bags (Fig. 4) for 48 h in the lab at 25

°

C and thenthe bags were removed and the plants were placed in a green-house at 20

°

C night/28

°

C day. Observations were made dailyfor disease symptoms.

Fig. 4. Inoculated plant in plastic bag acting as a humidity chamber.

Fig. 5. Inoculated fern showing advanced leaf necrosis.

Fig. 6. Rhizoctonia mycelial strand showing foot cell.

Fig. 7. Healthy plant

130

Proc. Fla. State Hort. Soc.

115: 2002.

Results and Discussion

The disease symptoms first appeared as water soaked spots10 mm in diameter. After 20 d, necrotic lesions became visibleon the inoculated plants. The spots enlarged to 25 mm or moreand then turned dark brown. A whitish mycelium grew rapidlyover the leaves, killing them (Fig. 5), and spread a mycelial webfrom leaf to leaf. Many small brown sclerotia and web-likemycelia were found on the leaves (Fig. 6), typical of the diseasesymptoms found on the infected nursery plants.

Rhizoctoniasolani

was consistently re-isolated from all the inoculated plantswhile no symptoms were observed on the uninoculated plants(Fig. 7). Thus Koch’s postulates were thereby fulfilled.

Literature Cited

Alfieri, S. A., Jr., K. R. Langdon, J. W. Kimbrough, N. E. El-Gholl, and C.Whelburg. 1991. Disease and Disorders and Plants in Florida. Bulletin

No. 14, Fla. Dept. of Agr. & Consumer Ser., Div. of Plant Industry, Talla-hassee, FL.

Chupp, C. and A. F. Sherf. 1960. Vegetable diseases and their control. RonaldPress, New York. 692 p.

Dwivedi, R. P. and S. C. Dubey. 1987. Web blight of

Eupatorium cannabinum

L. caused by

Thanatephorus cucumeris

. Indian Jour. Mycology and PlantPathol. 161:309.

McMillan, R. T., Jr., H. Vande Hei, and W. R. Graves. 1994. First report of webblight caused by

Thanatephorus cucumeris

on

Cupaniopsis anacardiopsis

inthe United States. Plant Dis. 78:317.

McMillan, R. T., Jr., H. Vande Hei, and W. R. Graves. 1994. First report of webblight caused by

Thanatephorus cucumeris

on

Sophora

tomentosa

in the Unit-ed States. Plant Dis. 78:317.

McMillan, R. T., Jr., M. Borek, and W. R. Graves. 1997. Web blight of dwarfHawaiian snowbush. Proc. Fla. State Hort. Soc. 110:370.

Pirone, P. P. 1970. Diseases and Pests of Ornamental Plants. Ronald Press,New York. 546 p.

Preston, D. A. 1968. Host index of Oklahoma plant diseases supplement,1948. Plant Dis. Reptr. 32:398-401.

Sharma, J. K. and K. V. Sankaran. 1984. Rhizoctonia web blight of

Albizia fal-cataria

in India. European J. of Forest Pathol. 14:261-264.

Proc. Fla. State Hort. Soc

. 115:130-133. 2002.

PESTICIDE MODE OF ACTION CODES TO AID ORNAMENTAL GROWERSIN DEVELOPING CONTROL PROGRAMS TO MANAGE PEST RESISTANCE

E

LZIE

M

C

C

ORD

, J

R

.

1

New College of FloridaNatural Science Division

5700 North Tamiami TrailSarasota, FL 34243-2197

J

AMES

F. P

RICE

AND

C

URTIS

A. N

AGLE

University of Florida, IFASGulf Coast Research and Education Center

5007 60th StreetBradenton, FL 34203

Additional index words.

ornamentals, insecticides, pesticides,insects, IPM

Abstract.

The development of resistance to pesticides hascaused problems in producing high quality, economicallycompetitive ornamental plants in Florida. Presently, there areapproximately 65 pesticide active ingredients involving atleast 25 different modes of action available for arthropod con-trol on ornamental crops in Florida. Resistance managementrequires growers to consider product mode of action as a ma-jor factor in rotational schedules. Mode of action information isnot usually found on product labels or in informational factsheets, thus preventing many growers from developing indi-vidualized control programs with emphases on arthropod re-sistance management. A coding system has been developedto identify pesticide modes of action. Pesticides of differentmodes of action should be selected to apply in a rotation to asingle arthropod community, thus avoiding selection withinthat community for resistant individuals.

1

Corresponding author.

Arthropod resistance to pesticides has been a problem inornamental crop culture since the early era of synthetic organicpesticides. A notable example of the problem comes from theleafminer,

Liriomyza trifolii

(Burgess) outbreaks of the 1970s.Leafminers were causing losses in chrysanthemum, other annu-al ornamentals (gerbera, aster, gypsophila, and many beddingplants) (Price, pers. comm.) and some vegetables (Parrella etal., 1981). Several effective pesticides including early organo-phosphates (methyl parathion), carbamates (oxamyl), pyre-throids (permethrin) (Robb and Parrella, 1984), and triazines(cyromazine) (Price, 1984), were identified for leafminer con-trol during the outbreak period, but pesticide efficacy was lostdue to resistance by the leafminer (Mason et al., 1987).

Methods to manage arthropods have included reducing theselection pressure of pesticides by emphasizing rotationsamong pesticides of different chemical classes and limiting re-peated applications of pesticides within identical ones. Thismethod is flawed in that pesticides of multiple chemical classessometimes compromise biological processes (mode of action)that are identical. Rotation between different chemical classeswith the same mode of action increases the selection pressureand can result in more rapid resistance. For example, tradition-al rotational schemes could allow rotations between organo-phosphate and carbamate classes, but both are acetylcholinesterase inhibitors that interfere with neural transmis-sion. Additionally, some pesticides within a single chemical classpossess different modes of action and rotation between thoseshould not increase the selection pressure to either. For exam-ple, carbaryl and fenoxycarb are carbamates, but the mode ofaction of carbaryl is to inhibit acetyl cholinesterase at the synap-tic cleft and fenoxycarb mimics juvenile hormone. Use of oneshould not necessarily affect the selection pressure of the other.