|Crop Knowledge Master|
green onion seedlings
Leaf blight and rot of green onion
TYPE: Kingdom: Stramenopila
Damping-off of green onion seedlings
Leaf blight and rot of green onion
In 1989, a detailed comparison was made of the species Phytophthora parasitica and Phytophthora nicotianae. Researchers concluded from these studies that both names were being applied to a single species. Since Phytophthora nicotianae was described first, in 1906, it has nomenclatural priority and all isolates of P. parasitica are now placed in Phytophthora nicotianae. Many growers are familiar with Phytophthora parasitica. The corrected name is now Phytophthora nicotianae.
Phytophthora nicotianae has a very wide host range and occurs on many species of plants. In Hawaii, it has been isolated from field crops such as eggplant, bean, parsley, watermelon, papaya and pineapple. In the U.S. It is a major pathogen of ornamentals, tobacco, and tomato. Farr et al lists over 70 hosts.
Green onions are grown commercially on all of the Hawaiian Islands except Lanai and Niihau, with Oahu having the majority of acres under cultivation. In 1991, Phytophthora nicotianae was found on an onion sample submitted to the Agricultural Diagnostic Service Center at the University of Hawaii at Manoa. Although the initial sample originated from the island of Oahu, occurrence of this disease on other islands is very possible, since this pathogen has been found on all islands. In Hawaii, Phytophthora nicotianae causes leaf and stem infections on Chrysalidocarpus palm, Catharanthus, Dendrobium, poinsettia, impatiens, sage, spathiphyllum, and many other plants.
On onion leaves, spots begin as small, irregularly shaped, water soaked lesions on the young and mature leaves (Fig. 1). In a few days these spots expand, girdling the leaf and causing the tissue above the infection point to wither (Fig. 2). As the infection progresses, healthy tissue is invaded, eventually killing the entire leaf (Fig. 3). Infected plants have a mix of healthy and withered leaves with some leaves showing a characteristic half infected-half healthy symptom (Fig. 4 and 5).
In addition to leaf and stem infection, Phytophthora nicotianae also causes root rots on: poinsettia, tomato, pineapple, and anthurium; crown and collar rots of: watermelon, protea, and African violet; and fruit rots of: tomato, papaya, and eggplant. This pathogen is especially serious because studies have shown it is not host specific. Disease on one plant can spread to adjacent plants of another species. The fungus produces sporangia (an asexual spore) that can either germinate directly, or can release up to 20 smaller, swimming spores called zoospores. Both types of sporangia and zoospores will send out fungal hyphae (hair-like strands) that penetrate onion leaves and cause disease.
Free moisture is necessary for pathogen spread. Sporangia need to be in water in order to germinate or release zoospores. Splashing water from raindrops or irrigation helps to move spores from infected plants to nearby healthy plants. The motile zoospores can swim in pools of standing water, drainage ditches or irrigation systems to infect plants far away from the original disease site.
Effective disease control begins with prevention and proper water management to minimize excess moisture on the plants. Avoid handling diseased and healthy plants at the same time since spores can be unknowingly transferred by workers during handling. Immediately remove and destroy any plants suspected of being diseased or showing signs of infection. Monitor plants regularly in order to detect disease outbreaks early.
Ridomil/Bravo 81W can be applied to green onion up to 14 days prior to harvest. Read label directions carefully and apply chemicals responsibly.
2. Raabe, R. D, I. L. Conners, and A. P. Martinez. 1981. Checklist of Plant Diseases in Hawaii. Hawaii Institute of Agriculture and Human Resources, College of Tropical Agriculture and Human Resources, University of Hawaii (Information Text Series 022).
3. Yoshimura, M.A., J.Y. Uchida, and M. Aragaki. 1985. Etiology and control of poinsettia blight caused by Phytophthora nicotianae var. parasitica and P. drechsleri. Plant Disease 69:511-513.
4. Statistics of Hawaiian Agriculture 1995, Hawaii Agricultural Statistics Service, P.O. Box 22159, Honolulu HI 96823-2159.
(c) Copyright J.Y. Uchida and Chris Kadooka
Department of Plant Pathology
University of Hawaii