The cause of leaf, petiole, collar, and cane rots of aglaonema in Hawaii was determined to be Fusarium subglutinans. A bacterial leaf rot was also investigated. Disease management advice is given

Uchida, J.Y.

Yahata, P.S.

ScholarSpace at University of Hawai'i at Manoa

.~BQIEfNo.lo6 Hawaii Institute of Tropical Agriculture and Human ResourcesCollege of Tropical Agriculture and Human ResourcesUniversity of Hawaii at ManoaA NEW AGIAONEMA FOLIAR BLIGHT AND CROWN ROTJ. Y. Uchida, Assistant Professor, Department of Plant PathologyP. S. Yahata, Research Associate, Department of Plant PathologyIntroductionSeveral species and cultivars of Aglaonemahave been grown in Hawaii for many decades.These species include A. commutatum, A.nitidum, A. crispum, A. costatum, A. simplex, A.pictum, A. rotundum, and A. modestum.'Fransher', 'Pseudobracteatum', 'Maculatum','Elegans', and 'Treubii' have been popularcommercial cultivars. More recently, 'SilverKing' (from the cross A. nitidum 'Curtisii' x A.pictum 'Tricolor'), 'Emerald Beauty', 'Pewter',and 'Silver Queen' have been Widely grown.In Hawaii, Aglaonema is a favorite shade-tolerant landscape or garden plant that is alsoenjoyed indoors. During several decades ofcultivation, few diseases have been observed onthis plant group in Hawaii. Unfortunately, theimportation of large numbers of aroids duringthe 1970s inadvertently introduced new bac-terial and fungal pathogens. In the mid-1980s, adevastating crown rot was obseIVed on cultivarsof A. commutatum at commercial nurseries onthe Big Island (Fig. 1). Similar specimens werereceived in 1987 from Oahu growers. Crop lossesat various nurseries were large, and researchinto the etiology of this disease was begun.Disease and SymptomsDiseased Aglaonema plants have leaf andpetiole (leaf stem) spots, sheath and stem (cane)rots, dead shoot tips, collar rots, and dead canes.On susceptible A. commutatuTTI cultivars, leafspots begin as faint, water-soaked tissue on theunderside of leaves, about one to two weeks afterartificial inoculation (Figs. 2 and 3). These spotsexpand and have dark centers with water-soaked edges. The top surface of the lesion is lessdistinct and slightly chlorotic. A week later,these spots are typically 10-30 mm in diameter,water-soaked with dark centers, and circular,elliptical, or irregular in shape (Figs. 2 and 3).On mature leaves, the spots have dark centers,large brown areas, and diffuse yellow borders(Fig. 4). On young unexpanded leaves, the spotsdevelop rapidly into large spots withoutextensive yellOWing. Petiole spots develop intoextensive rots, causing the leaves to collapse(Fig. 5).Stem or cane rots are dry, shriveled, andbrown, gray-green, or greenish black (Fig. 6).Associated with these stem rots are Wilting,yellow, or dying leaves attached to Olive-green toyellow petioles. Internally, diseased canes arebrown and spongy with darkened vascularelements. These darkened elements extend intothe firm, healthier sections of the cane (Fig. 7).Fungal sporulation is common on diseasedtissue and appears as white, crusty masses ondead tissue (Fig. 6).On intact plants, young tissue is more sus-ceptible to infection than older, mature tissue.Mature leaves are commonly infected at the leafedge, and disease development is rapid onceinfection occurs. Wounds are not needed forfungal penetration, but wounded tissue is verysusceptible.Plants with leaf, petiole, and stem rots, butwith little root rot, are characteristic of field-collected disease specimens. Repeated inocula-tions demonstrated that roots are relativelyresistant to infection (Fig. 8). Small brownlesions or spots are fonned on roots and expandslowly after inoculation of healthy plants. Incontrast, large rotted sections of the root systemare always associated with cane or collar rots.Causal Organism and SpreadA Fusarium species subsequently identifiedas F. subglutinans was consistently associatedwith diseased Aglaonema plants collected atseveral nurseries. Pure cultures of the suspectpathogen were obtained from diseased plants,and representative cultures from four com-mercial nurseries were studied. Pathogenicity ofF. subglutinans was confirmed by inoculationsof healthy plants with spores of F. subglutinans,12Fig. 1. Aglaonema plant from a commercial nursery with dead andyellow lower leaves, stem rot, and collapsed canes.Fig. 2. Early leaf symptom on Aglaonema caused byFusarium subglutinans. Note the young water-soakedlesion and older lesion with a dark center. Lowersurface of the leaf is pictured.Fig. 3. Leaf spotting of Aglaonema caused by Fusarium subglutinans.Note the young chlorotic lesion and older lesions with dark centersand chlorosis. Top surface is pictured.Fig. 4. Expanding leaf spots of Aglaonema caused by Fusarium subglutinans.Fig. 5. Petiole and leaf spots on Aglaonema causedby Fusarium subglutinans.Fig. 6. Lower petiole. cane. and collar rot of Aglaonema causedby Fusarium subglutinans. Note the a6undant fungalsporulation (white masses) on rotting tissue.3Fig. 7. Internal symptoms of Aglaonema cane rot caused byFusarium subglutinans.Fig. 8. Representative plant after root inoculation. Cane rotseventually develop, but only a few root lesions are formedtwo months after inoculation.Fig. 9. Representative plants one month after collar and root drench inoculation with Fusarium subglutinans.4Fig. 10. Effects of contrastinginoculum levels. Top row:Plants sprayed with 100spores/mi. Bottom row: Plantssprayed with 1,000,000spores/mi. 5 mI == 1 teaspoon.Fig. 11. Bacterial rot of Aglaonema leaves.5Fig. 12. Aglaonema 'Silver Queen' with limited lesiondevelopment on yOWlg leaves after inoculation.reproduction of disease symptoms (Fig. 9), andre-isolation of the fungus.Large numbers of conidia (asexual spores) ofF. subglutinans are produced on diseased planttissue. These spores are spread to healthy planttissue by splashing water, indiscriminate han-dling of infected and clean plants, contaminatedplanting and trimming tools, and probably byinsects or slugs.The development of airborne sexual spores,or ascospores, is known to occur in F. sub-glutinans (= Gibberella subglutinans), andascospores were observed in an early diseasespecimen. Ascospores have not been recoveredin recent specimens and have not been seen inculture. Further studies on this disease phase arecontinuing. At present, conidia have beendocumented as the primary agent for pathogenspread.6Fig. 13. Aglaonema 'Silver Queen' with a bllght of the spearleaf after inoculation.Fusarium subglutinans (= Fusarium monili-forme var. subglutinans) has been recorded onpineapple and banana in Hawaii. This funguscauses severe diseases of pine, maize, and sor-ghum elsewhere in the United States. This is thefirst record of F. subglutinans as a pathogen ofAglaonema or any aroid. There are few recordsof Fusarium on the Araceae. Fusarium solani isa reported pathogen of Diejjenbachia, and aFusarium species associated with Aglaonemahas been recorded in Florida.ControlResearch on the control of this disease hasrecently begun, and until specific recommenda-tions can be made, reliance on general diseasecontrol principles is necessary to reduce diseaselevels.Sanitation. Prompt removal of diseasedplants and new lesions on otherwise healthyplants will reduce the inoculum or spore level inthe crop. A strong effort to reduce inoculumlevels by removing diseased plants and plantparts should reduce disease significantly.Experimentally, plants sprayed with high sporelevels were severely diseased and dying six toeight weeks after inoculation, while plantssprayed with low spore levels were rarelydiseased (Fig. 10).Diseased plants should be incinerated,buried in deep pits, or removed from the nurserysite. Although experimental information islimited, it appears that conidia of F. sub-glutinans can be airborne, making earlyremoval and destruction of diseased plant tissueimperative. Fusarium is able to survive manymonths in dead plant tissue, potting media, andbench and ground debris. The fungus will alsosurvive in soil under benches or in areasreceiving drainage water from contaminatedcrops. Fungal spores are also harbored on pots,trays, labels, shears, knives, gloves, andclothing.Nonmetallic items, benches, and concretewalkways can be disinfested with a 10 percentsolution of liquid household bleach or withquaternary ammonium compounds. Alcohol orheat can be used for metallic tools. Infested soilunder and around contaminated benches can bedrenched with a quaternary ammonium com-pound and covered with a layer of sand or gravelto reduce spread by splashing.Ideally, AglaoTlelna should not be grown onbenches previously contaminated by diseasedAglaonema. Using a nonsusceptible alternativecrop interrupts the disease cycle, and rell1ainingFusarium spores eventually die.Moisture control. The growth and repro-duction of Fusarium species is favored by highmoisture. Increasing the humidity in theAglaonema growing environment will increasethe rate of infection, disease development, andfungal sporulation.Large numbers of spores are produced onalmost all lesions ll10re than 20 111111 indiameter. Fungal sporulation is especially highon diseased canes and petioles. These sporessplash onto healthy tissue on the same leaf oradjacent leaves, germinate, penetrate the host,and cause new lesions.In Hawaii, tropical conditions are ideal forcommercial aroid production but, unfortunate-ly, are also ideal for disease development bymany fungal and bacterial pathogens (Fig. 11).Although Aglaonema thrives in high humidity,reduced humidity levels need to be ll1aintaineduntil disease levels are lowered or the fungus iseradicated. Drip irrigation will reduce diseasespread by eliminating the splashing effect ofoverhead irrigation and shortening the leaf-wetness period.Saran houses provide no protection fromfrequent and prolonged rains. Solid-coveredgreenhouses are highly recommended, especial-ly for new, expensive cultivars or cultivars thatare very susceptible to fungal and bacterialdiseases. In a dry environment, spores do notgerminate and eventually desiccate and die.Maintaining disease-free stock. New stockshould be started from disease-free plantsobtained from a clean nursery or by growingtissue-cultured plants. If these are unavailable, asmall group of clean stock plants should beestablished. A clean Aglaonema stock can bestarted from diseased plants by (1) taking tip orcane cuttings from the least contaminated ordiseased plants (healthy Aglaonema canes arecompletely white internally: light brown to darkstreaks in the cane indicate disease), (2) peelingor trimming all leaves and sheath material, (3)thoroughly washing all plant surfaces inrunning tap water, (4) drenching with a surfacedisinfestant (e.g., a 20 percent solution ofhousehold bleach), (5) air drying for 12-48 hoursin a clean area, (6) planting individual canes inseparate pots using clean media, (7) growingplants in a glasshouse or under solid cover, (8)irrigating new plants by drip methods or hand-watering to avoid wetting the cane or foliage,and (9) closely monitoring new cuttings, withimll1ediate removal and destruction of all plantswith SymptOll1S of any disease. Repeating thisprocedure with canes taken from plants estab-lished by this method will further improve thepurity of the stock plants. Disease-free stockplants of Dieffenbachia were successfullyestablished by similar methods.Propagation from severely diseased stockplants is a poor practice and will only perpetuatethe existing problem. If this is unavoidable, e.g.,to save rare cultivars, the follOWing remedialprocedure is suggested: (1) move plants to a dryenvironll1ent where moisture is controlled, (2)remove all severely diseased or dry tissue (whengreen leaves with lesions are cut off, the sheathmaterial should be carefully peeled away afternatural abscission of the sheath has occurredwithin a few weeks: fresh wounds on the cane,caused by the removal of leaf sheaths before thesheaths are dry, provide an entryway for thefungus), (3) make two or three applications of afungicide such as mancozeb to reduce thenumber of viable spores on the external plantsurface, (4) avoid wetting the foliage, and (5) in7one to three months, if there are no new leaf orcane infections, take cuttings and treat as de-scribed earlier.Use of resistant cultivars. In preliminarystudies, 'Silver Queen' and 'Emerald Beauty'showed significant tolerance to F. subglutinans.Mature leaves were very resistant, even toinoculations with high spore levels. Small,brown leaf spots about 1-3 mm in diameterdeveloped on some young expanding leaves of'Silver Queen', but these spots failed to enlarge(Fig. 12). The unexpanded spear or new leaf wasoccasionally blighted after inoculation, butthese lesions rarely expanded as the leafmatured (Fig. 13).Minor infections on resistant or highlytolerant cultivars should be recognized andremoved from the plant. Additionally, tolerantcrops should not be grown adjacent to sus-ceptible cultivars, because minor lesions on'Silver Queen', for example, may provide theinoculum to start epidemics on susceptiblecultivars of A. commutatum. Accurate diagnosisand identification of the pathogens involved arecrucial to implementing cost-effective controlmeasures, including production 'of the mostsuitable cultivars. By combining crop tolerance,nursery sanitation, pathogen eradication, andmoisture control, effective control of Aglao-nema foliar blight and crown rot can beobtained and high quality plants can beproduced.This research was supported in part by the u.s. Departmentof Agriculture under CSRS Special Grant No. 89-34199-4420.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. 02/938

A New Aglaonema Foliar Blight and Crown Rot

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A New Aglaonema Foliar Blight and Crown Rot

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