Characteristics and control methods for this economically damaging leaf pest are described

Sipes, Brent S.

Uchida, Janice Y.

ScholarSpace at University of Hawai'i at Manoa

Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June30, 1914, in cooperation with the U.S. Department of Agriculture. Charles W. Laughlin, Director and Dean, Cooperative Extension Service, CTAHR, Universityof Hawaii at Manoa, Honolulu, Hawaii 96822. An Equal Opportunity / Affirmative Action Institution providing programs and services to the people of Hawaii withoutregard to race, sex, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or veteran status.Cooperative Extension Service Plant DiseaseOct. 1998PD-13Foliar nematodes are microscopic roundwormsthat parasitize many types of plants in Hawaii,including orchids. These nematodes are active swim-mers and commonly move over wet plant surfacesto infect plant tissues, causing leaf rots and inhibit-ing growth of floral buds. This publication describessymptoms of diseases caused by foliar nematodes inorchids and recommends disease prevention and con-trol practices.Foliar nematodes are common in warm, moistenvironments. In Hawaii, many other plants besidesorchids are hosts of these pests, including Africanviolet, anthurium, bird’s nest fern (Asplenium sp.),laua‘e fern (Polypodium sp.), coleus, chrysanthe-mum, fireweed (Erechtites hieracifolia), straw-berry, hibiscus, hydrangea, garden balsam (Impa-tiens balsamina), pentas, rice, tuberose, tree mari-gold (Tithonia sp.), and zinnia. Major commercialproblems have occurred in field-grown tuberosecrops and greenhouse-grown ferns. In both cases,leaf rots were the primary symptom of nematodeinfection.In the orchid family, foliar nematodes have beenobserved in Hawaii on cattleya, cymbidium, dendro-bium, oncidium, and vanda. The species of foliarnematode on vanda and Dendrobium nobile hasbeen identified as Aphelenchoides besseyi, whereasAphelenchoides fragariae has been found on on-Foliar Nematodes on Orchids in HawaiiJanice Y. Uchida and Brent S. Sipes, Department of Plant Pathologycidium. Diseases of vanda blossoms, dendrobiumfoliage, and oncidium have cost orchid growersthousands of dollars in unmarketable products. Be-cause many people are not aware of these patho-gens and their symptoms and control, diseasescaused by foliar nematodes often remain undiag-nosed for years, and losses are increased.Disease symptoms caused by nematodesFoliar nematodes enter plants through wounds ornatural openings such as stomatal pores on leaves.The method of entry varies among host plants.Nematode feeding and movement within the plantcauses damage by disturbing tissues. This damagebecomes visible as tissue discoloration, formationof rots, or inhibition of growth in young buds andshoots.On vandas, foliar nematodes infect immaturebuds and prevent flower formation. Diseased budsbecome yellow and abscise (drop off) or becomeblackened and adhere to the spike (flower cluster).Many buds may be killed, and the entire spike maybecome blackened and shriveled. Infection of de-veloping flowers causes rots of petals and sepals,and petals become deformed when the diseasedareas fail to expand and unfold normally.PD-13 Foliar Nematodes on Orchids CTAHR — Oct. 19982On oncidiums, infected buds are slightly yel-low and the sheath covering the buds becomes yel-low (Figure 1, D3). Eventually the buds dry andbecome deformed as they fail to develop (Figure 1,D1 and D2). Infected flower spikes are short andbarren. Dark, elongated streaks develop on leafsheaths covering the bulb (Figure 2). Very thin,black scar tissue forms on leaf sheaths and spikes.These scars represent areas of host tissue that havebeen damaged by nematode movement and feed-ing. The dead cells become blackened.Commercial dendrobium cultivars are also at-tacked by foliar nematodes. The typical symptomsare large, irregular, slightly yellow, olive-greenblotches on mature leaves (Figure 3). The blotchescan occur anywhere on the leaf and become brownas the disease spreads. The epidermis (skin) of theCompare the oncidium flower spikes infected with foliarnematodes (D1, D2, and D3) with a healthy spike (H). Notethat the infected spikes D1 and D2 have dry sheaths andundeveloped floral buds. On infected spike D3, the sheathis slightly yellowed and separated from the stem.This pseudobulb of an oncidium orchid infected with fo-liar nematodes has a long, dark streak on the base of theleaf sheath.2D1 D2 D3 H13PD-13 Foliar Nematodes on Orchids CTAHR — Oct. 1998leaf remains smooth and unbroken, in contrast tofungal infections, which typically destroy the epi-dermis or cover the leaf surface with fungal spores.Leaves infected with foliar nematodes  turn yellowas the brown blotches increase in size (Figure 4).These leaves then die and abscise.Leaf symptoms of infected Dendrobium nobile be-gin with small, dark, greasy areas, frequently on theleaf edges. These areas expand into larger dark spotsthat eventually have a wide chlorotic (yellow) areasurrounding the spot. The entire half of the leaf that isinfected may become yellow. Complete leaf yellow-ing is followed by necrosis (leaf rot) and leaf drop.On cymbidium leaves, foliar nematode infec-tion is characteristized by long, dark streaks (Fig-ures 5 and 6), whereas large, dark rots occur on cat-tleya leaves.Foliar nematodes also attack the roots of manyorchid plants. Plants thus infected may have reducedvigor and be stunted. These symptoms can be con-fused with poor nutrition or fungal diseases of theroot system.On this green dendrobium leaf with a blotchy discolora-tion, the discolored area is infected with nematodes. (Theblack scar and white blemish are not part of the symp-tom.)3This older dendrobium leaf infected with foliar nematodeshas a brown, blotchy area near the tip of the leaf. (A rect-angular leaf section was removed for microscopic exami-nation.)4PD-13 Foliar Nematodes on Orchids CTAHR — Oct. 19984Foliar nematode biologyFoliar nematodes are microscopic roundworms from0.4 to 1.2 mm long. Like other plant-parasitic nema-todes, Aphelenchoides species have a pointed stylet(spear-like organ), which is used to puncture cell wallsand feed on plant cells. They feed on cells within theplant, often moving internally between cells, andmultiply by producing eggs.When conditions are moist enough, foliar nema-todes swim by undulating their long, slender bod-ies. This ability allows them to migrate from theroot zone to floral buds at the tips of plants.Foliar nematode spread and survivalIn most potted orchids, foliar nematodes reside inthe roots of the plant. They feed on root tissues andmultiply within infected roots. They may also feedon fungi in the rhizosphere. On vandas, the nema-todes reside between leaf sheaths and can survivelong periods with little or no moisture. They maysurvive in dried buds and spikes for many months.During wet periods caused by rain, overhead irri-This diseased cymbidium plant has a darkened pseudo-bulb and prominent, dark brown leaf streaks caused byfoliar nematodes.Close-up of leaf streaks caused by foliar nematodes oncymbidium.5 65PD-13 Foliar Nematodes on Orchids CTAHR — Oct. 1998gation, or high humidity followed by condensation,foliar nematodes emerge from diseased tissue orroots and swim in films of water on the externalsurface of the host plant, moving rapidly to the up-per parts of the plant.The numbers of foliar nematodes in infectedplant tissue can be high. In moist weather, nema-todes are commonly present on the plant surface,especially near rotted areas. Water droplets on in-fected vanda plants may each contain 100–200nematodes. Nematodes are easily splashed from oneplant to another. They also migrate from one plantto another if the environment is moist and plantsare kept close together. Foliar nematodes can becarried in drainage water also. Most long-distancemovement of foliar nematodes occurs when infectedhost plants, contaminated soil, or infested pottingmedia are transported to new locations.Foliar nematodes can remain in potted plantsor in field soil for long periods. They feed on rootsand fungi in the root zone. They have a wide hostrange and can spread to many different kinds ofplants. For example, the foliar nematode Aphelen-choides besseyi from diseased vanda will infectchrysanthemum, African daisy, Shasta daisy, aster,cornflower (Centaurea cyanus), zinnia, dahlia, glox-inia, and coleus. This nematode also infects weedsin Hawaii, such as Emilia species and Asiatic pen-nywort (Centella asiatica).Disease control for foliar nematodesControl options for nematodes on orchids are lim-ited. Chemicals that eradicate these pathogens fromorchid plants or have therapeutic value are not le-gally available for use. Crop damage resulting fromplant-parasitic nematodes on other plants is mini-mized or prevented by soil fumigation (e.g., pine-apple), use of tissue-cultured plants (e.g., bananaand anthurium), or use of resistant varieties (e.g.,tomato).PreventionPreventing crop contamination is the key to diseasecontrol. When crops are started from clean seeds ortissue-cultured plantlets, they are generally free ofpathogenic nematodes. Thereafter, contaminationmay originate from poor sanitation practices thatallow nematode introduction. Some likely contami-nation sources and suggested methods of preven-tion include the following.® Non-orchid plants may be diseased or infestedwith foliar nematodes. Growers should keep onlyclean orchids in the nursery.® Other orchid plants introduced to an orchid pro-duction area or hobbyist collection may be diseasedor infested. Newly acquired orchid plants shouldbe kept in an isolated area and monitored frequentlyfor disease symptoms for at least six months.® Cinders that have been gathered near infectedplants may be infested with nematodes. Cindersshould be obtained from a cinder source with mini-mal vegetation.If cinder quality is uncertain, it can be fumigatedwith products such as Vapam® (follow the label di-rections carefully). Also, exposure to high tempera-tures (above 40°C, 104°F) for 30 minutes kills plant-parasitic nematodes; higher temperatures require lesstime. When using heat to eliminate nematodes, thecenter of the cinder pile must reach the desired tem-perature for the required amount of time.The same sanitation considerations given forcinder apply to all other ingredients of the pottingPD-13 Foliar Nematodes on Orchids CTAHR — Oct. 19986mixture, although it is not known if foliar nema-todes are present in hapu’u.*® Potting media should not be reused because itharbors many pathogens and pests that will causeplant diseases. Never save old potting media andmix it with new materials.® The soil of the orchid field can be a source ofplant-parasitic nematodes. In preparing the field forcut flower production, weeds and other vegetationshould be killed and removed. Weed mats coveredwith a thick layer of gravel or cinders can be usedto reduce contamination of the root zone. Gravelbeds used to plant orchids should be raised abovethe ground level to prevent root growth into the soilbelow the gravel. Remove weeds immediately.The field must be properly contoured to allowwater to drain toward the aisles and out of the field.Water that drains across plant rows may carry plant-parasitic nematodes and fungal pathogens to healthyplants.For growers producing potted plants for export,the crops must be grown on raised benches. If anew planting area was previously covered with veg-etation, clear the ground and cover it with weed matand a thick layer of gravel. In existing nurseries,benches with infested plants should be cleared ofplants, cleaned, drenched with a 20% solution ofhousehold bleach, and allowed to dry. The groundbelow these benches should be covered with a thicklayer of fresh gravel or cinder. Old gravel and soilshould be removed from under the benches if theaddition of new gravel raises the ground level andthe benches become lower than quarantine regula-tions allow.® Pots, tags, stakes, trays, and tools should becleaned with soap and water and surface-disinfestedwith a 10% solution of household bleach for 10minutes after each use. Bleach solutions should beprepared immediately before use and should not bestored. A few drops of detergent added to the bleachwill reduce air bubbles on the surface of pots thatinterfere with nematode kill.® Snails, slugs, toads, frogs, mice, and other smallanimals can move nematodes into clean fields oronto clean benches. Prevent entry of pests such astoads and eliminate slug, snail, and insect infesta-tions. A dry barrier of gravel without weeds or plantsshould surround the greenhouse to discourage en-try of these pests. The barrier should be at least 5 ftwide. Weeds near the field will harbor many pestsand make control difficult.Tires of vehicles and footwear of employeesmay carry nematodes during moist periods, particu-larly if plant debris is scattered over the roadway orin aisles.Therapeutic disease control measuresFor orchid plants with existing foliar nematodeproblems, the following procedures are suggested.• Control moisture. Keep the growing environ-ment as dry as possible. Early morning dew can pro-vide sufficient moisture for nematode migration.When the weather is dry, incidence of diseasescaused by foliar nematodes drops to nearly zero.Migration and subsequent infection by foliar nema-todes is prevented without water. Solid-coveredgreenhouses are strongly recommended. Air move-ment within the field or greenhouse should be maxi-mized to encourage evaporation and leaf dryness.Trees and shrubs surrounding the field should betrimmed to allow for good air movement. Watering*The hapu’u plant (Cibotium glaucum) produces thick, dark,fibrous roots around a hard central stem, and its fibers are com-monly used in the culture of orchids, anthuriums, and other plants.7PD-13 Foliar Nematodes on Orchids CTAHR — Oct. 1998should be conducted early in the day and not late inthe afternoon. Drip irrigation is preferable to over-head systems.• Reduce levels of nematode inoculum. Removeall diseased leaves, spikes, and other infected plantparts from the field. This will prevent plant-to-plantspread of the nematodes. Send infected plant partsto the county dump or incinerator. Do not pile dis-eased plants at the nursery, because these piles willserve as a source of nematodes that will infecthealthy plants.• Quarantine and isolate. If only a few pottedplants are infected, remove them from the maincrop. Small diseased plants should be discarded. Alldiseased plants will serve as a continuous source ofinoculum. If plants are planted in the ground, re-move infected plants and discard them from thenursery. Do not replant in the same spot for at least6 months.• Keep growing areas weed-free. The area sur-rounding and within the crop should be kept free ofweeds and other plants. The host range of foliar nema-todes is wide, and weeds may harbor the nematode.• Disinfest plants. Hot-water treatment (45–50°C,113–122°F) has been used for other crops. Heattreatment can be used to kill nematodes in somestock plants, although the effective temperature fornematode eradication in orchids such as dendro-biums is not known. In experiments with vanda cut-tings, hot-water treatment for 15 minutes at 46°C(115°F) eliminated nematodes. At higher tempera-tures (15 minutes at 49°C, 121°F), growth of thevanda cuttings was set back for a month, but short-ening the soaking time to 5–10 minutes at 49°Ckilled nematodes without injuring the plants. Othertypes of orchids need to be tested to find the effec-tive temperature and time required to disinfest them.Plants that survive the heat treatment must be evalu-ated for several months to ensure that nematodeshave been eliminated.• Use new, clean fields. For commercial growerswith large numbers of infected or suspect plants, agradual conversion to new fields in a new locationusing new plants is highly recommended. This isespecially important if solid-covered greenhousesare not being used to control moisture. The newcrop should be grown at a different site or in a green-house separate from the infected plants. Clean plantsplaced in a separate section of an infested green-house generally become infected.Education of staffSuccessful disease prevention and control are highlydependent on good education. If employees under-stand how nematodes move and reproduce, thendisease control methods become more effective.Without a good understanding of how these micro-organisms spread, it is easy for a routine action, suchas leaving a watering nozzle on the ground, to causespread of nematodes. With good education, the rulesof sanitation and procedures to prevent contamina-tion will become “logical” and easier for employ-ees to remember and follow.Tremendous effort, commitment, and dedica-tion are needed to develop new, clean fields. A setof guidelines can be developed for individual grow-ers in consultation with CTAHR researchers andextension agents. Check with your local Coopera-tive Extension Service county agent for additionalinformation.

Foliar Nematodes on Orchids in Hawaii

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Foliar Nematodes on Orchids in Hawaii

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