hawaii institute oftropical agriculture and human

.~~5QIEf No. 093

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

FOLIAR BLIGHT OF LEEA CAUSED BY A PHYrOPHTHORA SPECIES

M. Aragaki, J. Y. Uchida, P. S. Yahata, and C. Y. KadookaDepartment of Plant Pathology

IntroductionLeea coccinea, known as West Indian holly,

is commonly used for landscaping in the tropics.Outside the tropics and especially in the temper­ate regions of the United States, it is a popularpotted plant. Red leea, also known as Rubra, isespecially prized for landscaping and is an easy,rapidly growing plant with striking coloraccents. In Hawaii, leea has been relativelydisease-free, except for a root and collar rotcaused by Calonectria and reported in 1981.Recently, a new leaf spot and blight causingsignificant crop losses was discovered on greenand red leea at two commercial nurseries onOahu.

The first disease specimens were from largepotted red leea plants received in 1987. A fewmonths later, young red leea seedlings, approxi­mately 2-3 inches (about 5-7 cm) tall, wereobserved with similar symptoms at anothernursery. In early 1990, an outbreak of thisdisease occurred on young potted green leea,approximately 8 inches tall, resulting in the lossof hundreds of plants in a commercial nursery.A study of this serious new disease was begun in1988 and is continuing in our laboratory.

Disease and SymptomsOn mature red leea, leaf spots were irregular

in shape, slightly lighter in color than thenormal reddish-purple leaves, and difficult tosee unless plants were inspected closely.(Fig. 1).From a distance, severely infected plants couldbe recognized by randomly Wilting leaflets andleaves, especially at the top half of the plant (Fig.2). Leaf blights and spots had irregular, darkgreenish-purple, water-soaked borders, especial­ly noticeable on the lower leaf surfaces. Lesionsdried into wrinkled, brittle, brown to gray-blacktissue. Older leaf spots and blights were purplishOlive-green to brown, appeared scalded, and wereaccompanied by considerable defoliation afterlesion expansion. The fungus also spread into

petioles and stems, formed dark purple-brown toblack lesions, and killed young branches.

On green leea, leaf spots and blights weremore distinct. Young lesions and activelyexpanding blights were grayish green to slightlybrownish green. As spots matured and dried, theolder areas turned dark brown, then black (Fig.3). Young plants were severely blighted, wilted,or killed, which resulted in a loss of more than70 percent of the crop at one nursery.

Causal Organism and SpreadA Phytophthora species, apparently new to

Hawaii, was recovered from diseased leea.Several collections were made from each nurs­ery where the disease occurred, and Phytoph­thora isolates were cultured. The Phytophthorasp. isolated from diseased tissue was similar toP. meadii, a known pathogen of rubber andbreadfruit in other parts of the world, and P.megakary~ a pathogen of cacao.

Pathogenicity of the Phytophthora sp.isolated from leea was confirmed by inocula­tions of red leea plants, using a zoosporesuspension prepared from pure cultures. Typicaldisease symptoms, ranging from leaf spots (Fig.4) and leaf wilt (Fig. 4, 5) to plant death, werereproduced and the fungus was re-isolated.

Fungal spores, or sporangia, are dislodgedand spread primarily by water droplets (e.g.,rain or overhead irrigation) and air movement.These spores land on healthy tissue, germinatewith moisture, and produce infectious hyphalthreads that penetrate the host and proliferatewithin the plant. Initial symptoms of thedisease appear as small spots two to three daysafter infection. These small spots eventuallyexpand into blights and kill various parts of theplant. The rates of spot expansion and blightdevelopment depend on moisture and tempera­ture. Free moisture and temperatures in the 68­86°F (20-30°C) range favor lesion expansion.New spores produced externally on diseasedtissue begin the cycle over again. On rare

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Fig. 1. Diffuse pale leaf spots on L. coccinea causedby Phytophthora sp. (see arrows).

Fig. 2. Healthy L coccinea on the left and infectedplant with diseased drooping leaves on the right.

Fig. 3. Irregular black leaf spots on green L.coccinea caused by Phytophthora sp.

Fig. 4. Gray-black active blights caused by Phytophthora on greenL. coccinea.

Fig. 5. Healthy L. coccinea on the left; inoculatedblighted. defoliating plant on the right.

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occasions, large masses of sporangia are some­times visible as white to creamy, crusty areas onthe diseased plant. More commonly, however,these small sporangia are too few to be observedby growers.

Long periods of free moisture from rain oroverhead watering favor the pathogen andencourage its spread. Water is needed for fungalgermination, penetration of the host, and rapiddi.sease development.

ControlThe Phytophthora sp. attacking leea appears

to be very persistent and continues to growwithin the host even when the environment isdry. Disease control should emphasize preven­tion rather than treatment of plants after thedisease has occurred.

Effective disease control after the crop hasbeen contaminated is possible only with aserious commitment to reduce disease levels byremoving and destroying all badly diseasedplants and infested debris. Diseased plantsshould be removed from the nursery or inciner­ated. At low to moderate disease levels, allinfected and suspect leaves, twigs, and stemsshould be removed and destroyed. Crops need tobe checked daily, and any new lesions must beexcised immediately before new spores areformed. Plants with obvious root rots should bediscarded. Nursery clean-up should be assignedto a specific individual who will not fertilize,transplant, or handle healthy plants in thesame day. Handling badly diseased plantscontaminates hands, clothing, and tools withfungal spores.

The growing environment should be kept asdry as possible. Good air circulation should beencouraged by fans, greater interplant spacing,bench design, and crop position within thegreenhouse. Without solid-covered greenhouses,disease control in contaminated crops is nearlyimpossible during prolonged wet periods.

The economics of attempting to salvageheavily diseased crops is questionable. If it isattempted, all dying or severely diseased plants

should be destroyed, and remaining plantsshould be cleaned thoroughly by removing allsigns of infection. It is important to recognizethat plants still harbor the pathogen, even afterclean-up and effective fungicidal treatment;using these plants in landscaping will contami­nate many areas in the state with this newPhytophthora.

Preliminary fungicide trials comparedchloroneb (Demosan) at 15 Ib/100 gal,phosethyl-Al (Aliette) at 5 Ib/100 gal, ethazole(Truban) at 12 oz/100 gal, and metalaxyl(Subdue) at 10 f1 oz/ 100 gal on green leea plantsapproximately 11/2 to 2 feet tall. Treated plantswere drenched with the fungicides, maintainedin the greenhouse, and inoculated a week laterwith a suspension of Phytophthora zoospores.Disease development was monitored in thegreenhouse over a period of three weeks. Bestresults were obtained with metalaxyl, althoughsome leaf spotting still occurred. Plants receiv­ing no fungicide were severely blighted anddefoliated within a week.

Foliar applications of mancozeb (Manzate200) will kill spores and prevent infection andsubsequent establishment of the pathogen ifinoculum or spore levels are low. A contactfungicide such as mancozeb is known to destroyonly a percentage of pathogenic spores; it doesnot eradicate all spores. Good nursery sanita­tion will keep spore levels low and greatlyenhance the effectiveness of this fungicide. Veryhigh disease levels make mancozeb Virtuallyuseless against this aggressive pathogen.

Since treatment of diseased plants providesonly limited benefits, effective control must beprOVided by preventive measures. ThiS includesidentification of clean seed and seedling sources,nursery sanitation, close crop monitoring, andprobably one or two fungicidal applications toprevent infections, especially in nurseriesexposed to this new Phytophthora.

The authors thank the Governor's Agriculture CoordinatingCommittee for financial assistance making this researchpossible.

Reference to a company or product name does not imply approval or recommendation of the productby the College of Tropical Agriculture and Human Resources, University of Hawaii, or the UnitedStates Department of Agriculture to the exclusion 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. HITAHR BRIEF 093 7/90

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