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of vinca or periwinkle (Catharanthus
Root and collar rots of vinca or periwinkle (Catharanthus roseus)
TYPE: Kingdom; Chromista
Foliar blight of vinca or periwinkle (Catharanthus roseus)
Root and collar rots of vinca or periwinkle (Catharanthus roseus)
In 1989 a detailed comparison was made of the species Phytophthora parasitica and Phytophthora nicotianae. Researchers concluded from these studies that these names were being applied to a single species. Since Phytophthora nicotianae was described first (1906), it had nomenclatural priority and all isolates of P. parasitica were placed in P. nicotianae.
Many growers are familiar with Phytophthora parasitica. The correct name is Phytophthora nicotianae.
Phytophthora nicotianae has a very wide host range and occurs on many species of plants. In Hawaii, this fungus has been recovered from fruit rots of tomato, eggplant, bean, papaya; root rots of onion, parsley, watermelon, strawberry, castor bean, hibiscus, poinsettia, Easter lily, dendrobium, protea, ti (Cordyline), Gypsophila, ilima, palm, macadamia; leaf spots and blights on spathiphyllum, vanda, dendrobium, and poinsettia; heart rots of ti (Cordyline), dendrobium, pineapple, African violet, and many others.
In continental United States, it is a major pathogen of tobacco, tomato, and ornamentals produced in greenhouses including Catharanthus.
Catharanthus roseus, commonly known as periwinkle or vinca is a common bedding plant in the U.S. It is popular in container gardens and new flower colors produced on compact cultivars, have increased its use in many public and home gardens.
Wilting plants are also very common (Fig. 1). These wilts begin from infected leaves or as a result of root infections. Leaf spots are initially small, irregular water-soaked areas easily visible on the undersurface of the leaf. These spots expand to rot the entire leaf.
Diseased leaves are gray-olive green to brownish. The rots progress from leaves to the stem, and sections or branches of the plant are killed. Yellow leaves and black leaf rots also occur near the stem. Movement of the fungus into the primary stem or into the larger stems causes wilting of the entire plant or large sections of the plant.
Root rots are not initially noticeable. Small feeder roots are infected and larger roots become diseased with time as the fungus feeds on and destroys the root system. Infected plants may be stunted, lack vigor, or appear to be suffering from nutritional deficiency. On smaller plants, movement of the fungus from the roots to the collar causes rots at the soil line. These plants, wilt rapidly since water movement up the plant is inhibited. On larger plants, collar rots may progress into a lower branch and sections of the plant are gradually killed. Infected branches commonly have no leaves and only dead twigs remain.
Phytophthora nicotianae produces microscopic, hyaline(clear) spores called sporangia. These asexual spores are papillate (have a pointed tip), nearly spherical (round), and release swimming spores when water is available. These swimming spores or zoospores, are able to swim for many hours. They are attracted to host tissue or organic matter. Zoospores eventually stop swimming, encyst (round-up and form a wall) and germinate. If the environment is not wet enough, sporangia germinate by producing fungal threads or germ tubes.
The fungus also produces a spherical spore with thickened walls. These are chlamydospores and do not release zoospores. They germinate by producing germ tubes. These spores are able to survive in soil for many months in the absence of the host.
Phytophthora nicotianae is heterothallic and has two mating types, A1 and A2. When both mating types are grown together, numerous sexual spores, called oospores are produced. Oospores are also thick-walled.
As with many tropical Phytophthora species, the fungus is favored by warmer temperatures (25 to 28 C) and moisture.
This pathogen produces sporangia on the surface of diseased tissue. The sporangia release zoospores when water is present and zoospores or sporangia are splashed to healthy leaves or other plants. On the host, the germ tube produced by the zoospores will penetrate the host. Penetration of the host and growth of the fungus in the host begins the infection process. Continued feeding and growth of the microscopic fungus results in rots and disease symptoms. Sporangia also produce germ tubes that penetrate the host epidermis and begin the disease process.
Chlamydospores are produced on the inside of diseased leaves and stems, which fall to the ground. The diseased plant tissue decomposes and chlamydospore remain in soil for months, accounting for the soil-borne characteristic of this fungus. Movement of contaminated soil or infected plants moves the pathogen great distances.
Use clean seeds and monitor seedlings for any sign of disease. Remove any diseased seedling immediately. Control the moisture levels at the nursery, landscaped areas, or home garden by: using covered greenhouses or benches, increasing air movement to reduce leaf wetness, preparing the soil by adding compost and insuring good drainage. Cleaning up all diseased leaves and plants if the disease occurs. Removal of diseased plants and plant parts reduces the amount of spores that spread disease.
Commercial nurseries should dump their crops if the disease is found in the seedling stage. If nurseries attempt to salvage their crops, surviving plants will have Phytophthora root rots and the pathogen will be spread to many locations. Gardeners and landscapers should check each batch of plants before purchases. If any plants are wilting or have collar rots, avoid all of them.
Fungicides are available that reduce disease levels. However, no chemical will eliminate this pathogen and the pathogen will always be present once it is introduced among susceptible plants. Mancozeb, sold as Dithane M45 is a good contact fungicide and can reduce the number of leaf spots. Metalaxyl, sold as Subdue is an effective systemic fungicide that will protect leaves and new shoots. Follow label directions when using any fungicide.
Aragaki, M. and J. Y. Uchida. 1978. A new papaya fruit rot in Hawaii caused by Phytophthora capsici. Plant Disease Reporter 62:765-768.
Nagata, N. M. and M. Aragaki. 1989. Etiology and control of Phytophthora leaf blight of golden-fruited palm. Plant Disease 73:661-663.
Uchida, J. Y. 1989. Foliar blight of Spathiphyllum caused by Phytophthora. HITAHR Brief No. 84. CTAHR Publication, University of Hawaii. 3 pp.
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.
COPYRIGHT: Janice Y. Uchida