Plant Disease July 2012 PD-90

Powdery Mildew of Papaya in Hawai‘i

Brennen Cunningham1 and Scot Nelson2 1Department of Natural Resources and Environmental Management 2Department of Plant and Environmental Protection Sciences

apaya (Carica papaya L.) is with lower rainfall, growers use a large, herbaceous plant na- drip irrigation. tiveP to tropical America. By 1616, Important diseases of papaya it was grown widely throughout in high-rainfall areas of Hawai‘i the warm climates of Central and include Phytophthora blight and South America, the West Indies, anthracnose, both of which cause southern Mexico, the Caribbean, severe fruit rots. Other plant patho- and Southeast Asia (Perseley gens, arthropods, and nematodes 2003). The raw pulp of the sweet attack papaya and may cause sig- yellow or reddish-orange ripe nificant damage (Constantinides fruits is eaten fresh, and the green, 2008). A persistent and common immature fruits are used in salads threat to both home gardeners and or cooked and used as a squash- commercial farmers is powdery like ingredient. mildew. This fungal disease of Hawai‘i is the leading produc- papaya leaves and fruit causes er of papaya in the United States, premature defoliation, reduced with approximately 526 ha (1,300 yields, and lower sales due to acres) harvested in 2009 (USDA downgraded fruit or fruit not fit to NASS 2010). Although Hawai‘i’s harvest. In this article we describe of papaya fruits and leaf papayas comprise less than 1% of petioles. the symptoms and disease cycle of the global market, the state’s sale powdery mildew on papaya and of fresh and processed fruit earned over US$11 million suggest integrated practices for effective management in 2010 (USDA 2011). This made papaya one of the top of the disease in Hawai‘i. crops in the state. Hawai‘i currently exports papayas to the U.S. mainland, Canada, and Japan. Pathogen The Puna district on the island of Hawai‘i grows Powdery mildew on papaya was first reported from Brazil 92% of the papayas in the state. With the area’s high in 1898. The disease now occurs almost everywhere pa- annual rainfall (280 cm/100 in) and rocky soils, condi- paya is grown, including Hawai‘i, Brazil, Taiwan, Puerto tions for this crop are optimum. The well-drained soil Rico, the Dominican Republic, Haiti, Jamaica, Trinidad, is important, as papaya is susceptible to root diseases, Florida, and Australia. especially in heavier soils with a high clay content. In Papaya is host to a dozen species of fungi that cause high-rainfall areas plants do not need irrigation once powdery mildew (Tsay et al. 2011). In Hawai‘i the disease the seedlings are established, while in areas in Hawai‘i is caused by the pathogen commonly known as Oidium

Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in co- operation with the U.S. Department of Agriculture, under the Director/Dean, Cooperative Extension Service/CTAHR, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822. Copyright 2011, University of Hawai‘i. For reproduction and use permission, contact the CTAHR Office of Communication Services, [email protected], 808-956-7036. The university is an equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i without regard to race, sex, gender identity and expression, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or status as a covered veteran. Find CTAHR publications at www.ctahr.hawaii.edu/freepubs. UH–CTAHR Powdery Mildew of Papaya in Hawai‘i PD-90 — July 2012

Powdery mildew negatively affects the marketability of Most of the fruits on a given plant may show symptoms papaya fruits. of powdery mildew. caricae (= cruciferarum), which was officially attack unripe fruits. The powdery mildew also appears renamed O. caricae-papayae (Species Fungorum 2012). on papaya petioles, pedicels, and peduncles. The infective conidia are hyaline and barrel-shaped mi- In the early stages of the disease, the undersides of croscopically, white and granular macroscopically, and leaves are speckled with small, water-soaked spots that usually produced in chains of five or more. become powdery patches of mycelium and spores. The infections are usually concentrated near the leaf veins. Biology and Ecology The white patches are from 1–6 cm in diameter, with Powdery mildew of papaya thrives in humid areas with corresponding yellowish-green spots on the upper leaf warm days and cool nights, such as north Kona on the surface. The mildewed areas grow in size and coalesce, island of Hawai‘i. Spores of the pathogen, which can causing increasingly severe yellowing between the veins. germinate in 10–12 hours, are mainly dispersed by wind In some cases the spore-forming mycelium will wrap currents. Low light levels, high humidity, moderate tem- around the leaf edge and grow on the upper leaf surface peratures (64 to 90°F, 18 to 32°C), and moderate rainfall and petioles. (60 to 100 in, 1,500 to 2,500 mm per year) enhance As the disease progresses, severely infected leaves disease development (Ooka 1993). Oidium caricae is become necrotic and appear scorched. They curl and an obligate parasite, needing a living host to complete fall from plants prematurely. Mildew on immature fruit its life cycle. begins as circular patches of white mycelium and spores that can coalesce and cover the entire fruit. As the fruit Symptoms ripens, the may disappear, leaving behind grey Powdery mildew infects papaya plants regardless of their scars. The scars restrict growth of the underlying tissue, age, but seedlings grown in greenhouses are especially resulting in deformed fruit. The deformed fruit is edible susceptible, with apical tissues often destroyed. The but has little or no value in the market. disease frequently infects immature leaves but can also

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Cultural Management Integrate the following cultural practices with the proper pesticides to achieve the best control of powdery mildew.

• Select a site for cultivation that has warm tempera- tures, low relative humidity, and no extreme differ- ences in daytime and nighttime air temperatures. • Plant rows parallel to the prevailing trade winds to increase air movement in the canopy. • Do not irrigate with overhead sprinklers. This in- creases the relative humidity in the plant canopy. Although powdery mildew is inhibited by water on papaya leaves and fruit, the resulting humidity promotes infection and disease development. • Periodically remove and destroy heavily mildewed leaves; do not compost the material. • Ensure adequate nutrition but avoid excessive nitro- gen, which can cause a flush of succulent, vulnerable plant growth.

Fungicides There are several fungicides registered for powdery mil- dew of papaya in Hawai‘i (see Table 1). Organic growers Powdery mildew is visible here on the underside of a pa- may use elemental sulfur and potassium bicarbonate, paya leaf. Sprays of contact fungicides such as elemental which are both OMRI listed. When using a fungicide, sulfur must cover this leaf surface. always read and obey the label.

Table 1. Pesticides registered for control of powdery mildew of papaya in Hawai‘i.

Trade Name Active Ingredient (%) Formulation Abound® Flowable Fungicide Azoxystrobin (22.9%) Suspension concentrate Prev-Am® Ultra Sodium tetraborohydrate decahydrate (0.99%) Emulsifiable concentrate Companion® (GB03) (0.03%) Emulsifiable concentrate StorOx® Broad Spectrum Hydrogen dioxide (27%) Soluble concentrate BiocoverTM Mineral oil (98%) Fluid Rally® 40WSP Myclobutanil (40%) Water-soluble packets Kaligreen® Potassium bicarbonate (82%) Water-soluble powder Drexel Sulfur 90W Elemental sulfur (90.0%) Wettable powder/dust Flint® Trifloxystrobin (50%) Soluble concentrate Procure® 480SC Triflumizole (42.14%) Soluble concentrate

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Acknowledgements Agriculture and Human Resources. http://www. The authors thank Fred Brooks and Ty McDonald of extento.hawaii.edu/Kbase/crop/Type/o_caric.htm UH-CTAHR for their thoughtful reviews of this manu- (accessed 8 June, 2012). script and Mike Kawate of UH-CTAHR for providing Perseley, DM. 2003. Diseases of papaya. In: Ploetz, additional information. RC (ed.) Diseases of Tropical Fruit Crops. CABI Publishing. Cambridge, Mass., pp. 373–403. References Species Fungorum. 2012. Oidium caricae-papaya. Edmunds, B, and Pottorff, LP. 2009. Powdery Mildews. Index Fungorum. http://www.speciesfungorum.org/ Colorado State University Extension no. 2.902. http:// Names/SynSpecies.asp?RecordID=335333 (accessed www.ext.colostate.edu/pubs/garden/02902.html (ac- 6 July 2012). cessed 8 June 2012). Tsay, J-G, Chen, R-S, Wang, H-L, Wang, W-L, and Weng Constantinides, L, and McHugh, J. 2008.The Pest B-C. 2011. First report of powdery mildew caused Management Strategic Plan for Papaya Production by Erysiphe diffusa, Oidium neolycopersici, and in Hawai‘i. Workshop Summary (December 2, Podosphaera xanthii on papaya in Taiwan. Plant 2005), Komohana Research and Extension Center, Disease 95:1188. University of Hawai‘i at Mānoa, Hilo, Hawai‘i. http:// USDA, National Agricultural Statistics Service www.ipmcenters.org/pmsp/pdf/HIPapayaPMSP.pdf. (NASS). 2011. Papayas. Number of Farms, Acreage, (accessed 8 June 2012). Utilization, Farm Price, and Value, State of Hawaii Kable, PF, Fried, PM, and MacKenzie, DR.1980. The 2006-2010.Hawaii Farm Facts: 1–5. http://www.nass. spread of powdery mildew of peach. Phytopathology usda.gov/Statistics_by_State/Hawaii/Publications/ 70:601–604. http://www.apsnet.org/publications/phy- Archive/xfar0711.pdf. (accessed 8 June 2012). topathology/backissues/Documents/1980Articles/ USDA, National Agricultural Statistics Service (NASS) Phyto70n07_601.pdf (accessed 8 June 2012). 2010. Crop Production. National Agricultural National Horticulture Board. Papaya Diseases. http:// Statistics Service (NASS), Agricultural Statistics nhb.gov.in/fruits/papaya/pap002.pdf (accessed 8 Board, U.S. Department of Agriculture. http://usda01. June 2012). library.cornell.edu/usda/nass/CropProd//2010s/2010/ Ooka, J. 1993. Oidium caricae. Crop Knowledge Master, CropProd-02-09-2010.pdf (accessed 8 June 2012). University of Hawai‘i at Mānoa, College of Tropical

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