Hawaii Institute of Tropical Agriculture and Human ResourcesCollege of Tropical Agriculture and Human ResourcesUniversity of Hawaii at Manoa

DRACAENA DECLINE AND ROOT ROT

J. Y. Uchida, C. Y. Kadooka, and M. AragaldDepartment of Plant Pathology

IntroductionThe genus Dracaena includes about 40

species of tropical treelike shrubby plants,many of which are popular prized species usedfor interior and exterior landscaping. In Hawaii,cultivars of D. Jragrans, D. deremensis, D.marginata, D. rejlexa, and D. sanderana areimportant for landscaping as well as for export.Dracaena surculosa and D. goldieana are alsogrown for indoor use.

In recent years, severe root rot and poorgrowth of Dracaena have occurred at commer­cial nurseries on Oahu and Hawaii. Commonly,diseased plants were D. deremensis 'Warneckii'and 'Janet Craig', D. Jragrans 'Massangeana',and D. marginata. This study was undertaken todetermine the cause of this serious problem, andearly findings are presented here.

Disease and SymptomsIn pots, diseased Dracaena is typified by

stunted yellow leaves, wilted green leaves, orexcessive numbers of dead lower leaves.Frequently, new leaves are small and affectedplants fail to thrive months after planting.Potted plants also die or exhibit poor Vigor,making removal necessary. Loss of a single canein a pot disturbs the planting design and sym­metry, thereby devalUing the pot, and reducestotal production. In the field, affected stockplants also grow poorly, although dead plantsare found less frequently.

Diseased root systems of potted Dracaena arereduced, brown, and necrotic. Larger roots withvarying degrees of necrosis are frequentlypresent, but smaller healthy, fibrous roots arefew or absent. In severe cases, all roots arenecrotic and no functional roots remain.

A cane rot has also been associated withplants exhibiting severe root rot. Frequently,the green epidermal layers at the base of theplant are killed and the cane becomes off-whiteto light brown internally.

Microscopic examination of the diseasedroots revealed spherical fungal oospores, or

sexual spores (Fig. 1); inflated, lobate fungalgrowth resembling sporangia, or asexual spores(Fig. 2); and hyaline (colorless), narrow fungalthreads, or hyphae, the vegetative stage of fungi.Diseased roots and cane specimens were washed,trimmed, and plated on sterile agar to isolatepossible fungal pathogens. An examination forparasitic nematodes was also made, but onlyoccasional root-knot nematode larvae wereobseIVed.

Causal Organism and SpreadA large number of fungal species was isolated

from various disease samples collected fromdifferent nurseries. These organisms includedThielaviopsis paradoxa, Fusarium spp., Cylin­drocladium sp., Pythium splendens, P.graminicola, other Pythium spp., Phytophthorapalmivora, Colletotrichum sp., Gliocladium sp.,and Kutilakesopsis sp. Pure cultures of all fungiisolated were established, and pathogenicitytests were initiated with some of these isolates.

Healthy 'Massangeana' plants were used forthese tests. Plants were established from tipcuttings rooted in vermiculite, grown in a 1: 1mixture of vermiculite plus Sunshine BlendR 4,fertilized periodically with OsmocoteR 14-14-14,and maintained in a glasshouse.

Fungi were indiVidually tested by applyingspore suspensions or macerated agar cultures tothe root system. Plants were monitored forsymptom development for two to three months.These tests demonstrated that six isolates ofFusarium, two isolates of P. palmivora, twoisolates of Pythium spp., and one isolate ofCylindrocladium sp. were nonpathogenic. Rootsystems of plants inoculated with the abovefungi remained healthy, as did the controlplants, which receive.d no fungi.

The single isolate of P. splendens tested wasalso nonpathogenic, but further testing of otherisolates is reqUired. Pythium splendens wasisolated in high numbers from decliningDracaena. It commonly occurs in soil and is aserious root pathogen of several economic crops,

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Figure 1. Photomicro~raphof spherical, thick-walledPythiwn. oospore within fDfectecl Dracaena root ceDe

Figure 2. Photomicrograph of lobate structures resemblingPythiwn. spoJ"lUlgia within root cells of Dracaena.

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Figure 3. Representative root systems of inoculated (left) anduninoculated (right) Dracaena plants.

such as papaya, cucumber, and certain orna­mental plants. Pathogenicity tests of other P.splendens isolates collected from Dracaena areneeded before this fungus can be discounted as afactor in Dracaena decline.

Thielaviopsis paradoxa caused slight vascu­lar cane discoloration extending from the cutend of the rooted Dracaena cane, indicating thatit is a weak wound pathogen. This fungus isknown to cause severe rots of sugarcane andpineapple seed pieces.

Severe root rot developed on plants inoculat­ed with P. graminicola (Fig. 3). Two months afterinoculation, roots were in' advanced stages of rotwith 70 to 100 percent of the root systemnecrotic or lost. Infected roots were soft and fellapart easily. Above-ground decline symptomswere also evident, with inoculated plantsproducing small, stunted new leaves. The olderleaves on inoculated plants were moderatelychlorotic, dull green, slightly flaccid, or wilted.

Pythium graminicola has been difficult toisolate from diseased Dracaena and is frequent­ly overrun by fungi such as P. splendens, Fusari­um spp., and bacteria in isolation plates. Thecommon occurrence of P. splendens and otherPythium spp. in diseased Dracaena has beenparticularly troublesome in these studies. Thesefungi grow aggressively in isolation plates,making slower growing pathogens difficult toisolate or obselVe. Many Pythium spp. occurregularly in soil and are saprophytic ornonparasitic. These organisms qUickly invadeplant tissue that has been destroyed or weakenedby other pathogens. Efforts are continuing toimprove detection of P. graminicola withindiseased tissue and to readily distingUish it fromP. splendens and other Pythium spp.

Control and SpreadLittle is known about Pythium graminicola

in Hawaii. It is known to cause root rot of cornand has also been reported on sugarcane. Itspathogenicity to Dracaena is a new record.Growth, multiplication, and spread of thisfungus is favored by high moisture andtemperatures of 24-30°C (75-86°F). Motilezoospores are produced in the presence of freewater, although typical laboratory procedures toconsistently produce abundant zoospores inaxenic (sterile) cultures have been unsuccessful.

Until more is known about specific controlmeasures, adherence to certain general prin­ciples will be necessary to manage this disease.With respect to fungal root diseases, the firstprinciple of control is to reduce or eliminateinoculum or fungal spore levels. Movement offungal spores spreads disease. Spore germina­tion is rapid with free moisture and is quicklyfollowed by fungal penetration of the host,resulting in disease establishment.

Lobate structures resembling Pythium spor­angia are formed in large numbers in roots ofinoculated plants. In water, these sporangia areknown to release motile zoospores that areattracted to and infect roots. Oospores are alsocommonly formed in roots of diseased plantsand tolerate a wide range of environmentalconditions, enabling the fungus to survive forlong periods in soil without the host. Theirpresence necessitates destruction of diseasedplants by incineration or removal of theseplants from the nursery. Avoid reusing mediafrom pots with diseased plants without pasteur­ization or sterilization. Oospores will quicklygerminate and repeat the disease cycle whenplants are grown in contaminated media.

The second principle is the production ofhealthy plants through selection and protectionof new clean cuttings. Pythium is a soil inhabi­tant, and the diseases it causes are mostly rootrots or diseases of tissue and organs in contactwith soil.

Stock plants ineVitably will have some soiladhering to stems and leaves. In problem fields,these soil particles are likely to harbor spores ofpathogenic fungi such as Pythium. Taking

cuttings as high above the soil line as feaSiblewill minimize introducing infective propagulesinto the propagation area. If cuttings must beharvested close to the soil, Pythium contamina­tion can be reduced or eliminated by scrubbingthe lower cane surface to remove the dirt, thenspraying with a dilute disinfestant such as a 10percent solution of common household bleach.Dipping is not recommended, as it promotesbacterial diseases of the cane and foliage.

Cleaning and disinfesting knives, saws, orother cane-harvesting tools and keeping themoff the ground during harvesting will reducecontamination. Harvested canes must be pro­tected from direct contact with the soil surfacein problem fields: a clean plastic tarpaulin maybe useful for this purpose.

In the greenhouse, removal or isolation ofdiseased Dracaena plants will minimize diseasespread to newly established plants. SincePythium spreads through water movement, goodfield and bench drainage are critical. Avoidbenches that drain horizontally, as fungalspores and bacteria can be easily spread fromplant to plant. Snails and slugs also contributeto the movement of Pythium spores by ingestingdiseased plant tissue with fungal spores andexcreting the spores. The germination of theseexcreted spores is also known to be enhanced bypassage through snails.

More studies are needed to resolve theDracaena decline problem. Pathogenicity stud­ies of other fungi such as Gliocladium and otherPythium species obtained from Dracaena mustbe continued, and pathogenicity to Dracaena ofP. graminicola from other hosts such as cornmust be determined. The occurrence of P.graminicola in fields adj acent to Dracaena isalso important to the epidemiology and controlof this disease. Studies are also continuing onchemical control and growth and reproductiverequirements of P. graminicola.

This research was supported in part by the u.s. Departmentof Agriculture under CSRS Special Grant No. 89-34199-4430.Reference to a company or product name does not implyapproval or recommendation of the product by the College ofTropical Agriculture and Human Resources, University ofHawaii, or the u.s. Department of Agriculture to theexclusion of others that may be suitable.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department ofAgriculture. Noel P. Kefford, Director and Dean, Cooperative Extension Service, College of Tropical Agriculture and Human Resources,University of Hawaii at Manoa, Honolulu, Hawaii 96822. An Equal Opportunity Employer providing programs and services to the citizens ofHawaii without regard to race, color, national origin, or sex. HITAH R BR IE F 103-05/92

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