Pest Management Guidelines
|Damping-Off and Leaf Spot of Schefflera Caused By Colletotrichum|
By: Janice Y. Uchida
Associate Professor, Department of Plant Pathology, CTAHR, University of Hawaii
Figures referred to in this document are unavailable at this time.
Schefflera actinophylla (formerly Brassaia actinophylla) is frequently used in tropical landscapes and as an indoor, potted plant. It is commonly known as the umbrella tree because of its palmately compound leaves, or as the octopus tree because its floral racemes appear tentacle-like. These lush, green-foliaged plants grow quickly, tolerate low indoor lighting, and require little maintenance. Excellent bonsai plants have been created with Schefflera in spite of the herbaceous nature of the young plants. Recently, novel leis have been made with the floral buds.
Diseases of commercially produced Schefflera were rare in Hawai'i before the 1970s. Leaf blight caused by Alternaria panax has become common in the last decade and crop quality has been drastically reduced at some nurseries. In addition, seedling damping-off caused by A. panax or Pythium splendens has been encountered by commercial growers. A recent epidemiological study of A. panax revealed a number of new diseases of Schefflera caused by Colletotrichum, which are discussed in this paper.
On Schefflera, Colletotrichum is primarily a pathogen of young tissue. Immature leaves are highly susceptible and new leaves less than 10 mm in length are rapidly blighted (Figures 1 and 2), while petioles remain healthy. Leaf spots produced by Colletotrichum are oval to circular, vary in size (generally 3 to 10 mm in diameter), are tan centrally, and have irregular brown edges (Figure 3). As leaves mature, spot expansion slows and most of the necrotic areas frequently fall out, leaving holes with a narrow border of infected tissue. Loss of blighted young leaflets or inhibition of normal leaf expansion in the tissue surrounding lesions leads to severe leaf deformities (asymmetry, curling, cupping, etc.).
Damping-off of young seedlings is common (Figure 4). Thousands of plants have been lost at nurseries where high-density planting of Schefflera seed is a common practice. Seedling diseases severely decrease production and, eventually, the quality of mature plants (Figure 5). Spots on cotyledons and leaves are small, oval to irregularly shaped, brown, and frequently have chlorotic borders. Blighted cotyledons and leaves defoliate readily. Collar rots begin as small, brown lesions that gradually expand, darken, and girdle the stem (Figure 6). Collar and root rots stunt or kill plants and topple seedlings. These symptoms are frequently confused with root rots and damping-off caused by P. splendens.
Plants that survive seedling infection are poor in vigor and quality. Production time and crop costs are increased. The pathogen persists on larger plants and causes foliar deformities, leaflet losses, and reduced growth rates.
Figure 1. Blackened and blighted young leaves of Schefflera caused by Colletotrichum.
Figure 2. Close-up of young leaflets blighted by Colletotrichum. A healthy leaflet is on the left.
Figure 3. Leaf spot on Schefflera leaf caused by Colletotrichum.
CAUSE AND SPREAD
The fungus causing leaf spots and damping-off of Schefflera has been identified as Colletotrichum gloeosporioides. Colletotrichum produces spores (conidia) on diseased plant tissue. Although the growth rate of the pathogen is restricted on mature leaves, many spores are produced on older lesions or leaf rots. Spores form on diseased leaf or stem surfaces and masses of spores can be seen as pink or white areas. Spores are also common on diseased seedlings and spore masses with gray-black flecks appear on the stems of dying seedlings. High humidity retained in the canopy of Schefflera seedlings favors pathogen growth and spore production.
Spores are spread by splashing water from rain or overhead irrigation. The fungus is also distributed by contact and can be carried on clothing, gloves, tools, insects, slugs, etc. The fungus spreads over longer distances through the transport of diseased plants and seeds.
Moisture is required for disease development. Fungal spores that land on healthy Schefflera plants germinate only with moisture. At least 12 hours of moisture is crucial for spore germination and penetration of the fungus into the leaf. Without sufficient moisture, the spores die within a few days or germinated spores die before host penetration occurs.
In pathogenicity trials, Colletotrichum spores sprayed on large, healthy plants or young seedlings at the cotyledon stage cause leaf spots, rots, deformities, and damping-off. Succulent tissue produced by rapidly growing plants is readily infested. The seed coat and embryo are also attacked by this pathogen.
Compared to the isolates of C. gloeosporioides collected from other hosts (anthurium, papaya, orchids, etc.), those from Schefflera have slightly shorter spores and produce orange colonies with white edges on potato dextrose agar. C. gloeosporioides isolates that are pathogenic to Schefflera cause little or no disease on papaya or anthurium, indicating a degree of host specificity.
Figure 6. Stages of collar rot on Schefflera seedlings caused by Colletotrichum.
Incidence and severity of leaf spots and foliar blights can be reduced by application of protective fungicides such as mancozeb (Dithane M45 R'). Once established, complete eradication of the pathogen is unlikely. Sanitation or removal of all dead and infected plant parts is crucial to disease management. Removal of diseased leaves reduces pathogen population levels, allowing maximum benefits of chemical sprays to be attained. Protection of young tissue from infection decreases leaf deformities and defoliation.
Moisture is required for disease development. Without moisture, few spores are produced, most fail to germinate, and many eventually die. Therefore, controlling moisture controls disease. Good aeration, drip irrigation, or timed water applications can reduce leaf wetness periods and decrease infection levels. Solid, covered greenhouses (clear plastic, glass, or fiberglass) are highly recommended for high rainfall areas. A small fiberglass house to protect seedlings and young plants from excessive rainfall will reduce disease levels on vulnerable plants.
Damping-off of young seedlings must be controlled culturally and cannot be controlled with chemical sprays. Colletotrichum can infest seed surfaces and infect the seed coat or embryo. Blemish-free seeds should be selected, dipped in a freshly prepared 10 percent solution of a household bleach (e.g., Clorox) with a few grains of detergent, and agitated for 1 to 2 minutes before planting. Seeds should be planted and maintained in a fiberglass house in an area without older plants. Since Colletotrichum may persist internally in infected seeds, each crop should be carefully checked for signs of seedling leaf spots or collar rots. Any infected seedlings and those around them should be removed immediately and discarded.
Given the difficulty of controlling the disease in contaminated seedling trays or pots, identification of clean seed sources is important. Growers who maintain stock plantings of mature Schefflera should: (1) apply fungicides to floral racemes to control infection of the seed; (2) harvest fruits/seeds only from the plant and not from the ground; (3) select blemish-free fruits; and (4) wash seeds, remove pulp, dip seeds in Clorox( 3 as previously described, air-dry, and then store. By using clean seeds, the disease can be eliminated.
The fungus survives in dead seedlings and other plant parts. Potting media or soil from diseased plants should not be saved or reused. Nursery employees should not handle diseased plants and then touch clean plants because spores are easily spread via hands and tools. If pots, trays, tags, or other products are reused, they should be washed and surface disinfested in freshly prepared 10 to 20 percent Clorox. Diseased plants and contaminated media should be removed from the nursery site or regularly buried (deep). Trash piles containing diseased plants will generate high inoculum levels of fungal or bacterial pathogens and also increase insect and slug infestations.
Education of all employees is important to the production of high-quality plants. Employees should be taught: (1) the causes and symptoms of diseases; (2) how diseases are spread; and (3) how diseases can be prevented and controlled.
The use of trade names is for the convenience of readers only and does not constitute an endorsement of these products by the University of Hawai'i, the College of Tropical Agriculture and Human Resources, the Hawai'i Cooperative Extension Service, and their employees. All pesticide users should consult the product label to insure that the desired crop use is included to insure compliance with state pesticide use laws. Materials and rates of chemical application listed herein are based on the latest information available at the time this publication went to press. Supplemental information will be disseminated as need arises. Information provided herein is for educational purposes only.
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