|Crop Knowledge Master||Fungi|
|Yellow spot and
blight of dendrobium leaves
Blue to purple flower spots on dendrobium flowers
TYPE: Kingdom: Fungus
Phylum: Ascomycota (teleomorh or sexual stage)
Traditional: Sphaeropsidales (The imperfect fungi)
Coelomycetes (Fungal spores produced in pycnidia)
Form genus: Phyllosticta
TAXONOMY: This fungus was previously known as Phyllostictina pyriformis.
DISEASE NAME: Yellow spot and blight of dendrobium leaves
Blue to purple flower spots on dendrobium flowers
Major hosts are in the orchid family, and belong to genera such as Brassolaeliocattleya, Cattleya, Cymbidium, Dendrobium, Epidendrum, Laelia, Laeliocattleya, Odontoglossum. Oncidium, Phalaenopsis, and Vanda. Phyllosticta capitalensis has also been reported on Guzmania and Nidularium of the bromeliad family in Florida.
In Hawaii, this fungus is common on many cultivars of dendrobium.
Widespread in Hawaii and throughout the world on orchids.
The fungus causes leaf spots on all hosts that are listed. On dendrobium, young leaf spots are typically yellow or black, circular, and small. These spots remain unchanged for many months (Fig. 1). However, the fungal growth within the leaf tissue eventually increases and tan to brown rots develop. Within the next week or two, the entire leaf is killed and the characteristic, black web-like pattern formed by this fungus on diseased leaves is observable (Fig. 2).
On dendrobium, flower spots are small, faint, and commonly blue or lavender on purple cultivars (Fig. 3). When the flowers are harvested or drop from age, a black rot rapidly covers the flowers in 1 or 2 days (Fig. 4).
Phyllosticta and other members of the fungal group Sphaeropsidales, produce small, black fruiting bodies (pycnidia) that contain asexual spores on the surface of infected tissue. These asexual spores or conidia are released when moisture is present and also require moisture to germinate. In addition, black fruiting bodies called perithecia, are also formed, shortly after pycnidia are produced. Perithecia produce sexual spores or ascospores. Ascospores are produced in sacs and there are 8, single-celled spores per sac for this fungus. When a fungus produces ascospores, it is classified as an Ascomycete and has a name that characterizes different fungi within the Ascomycetes. Thus, when these perithecia are formed, the fungus can be classified as a Guignardia species because the perithecial and ascospore characteristics match those of Guignardia.
The fungus is easy to culture on agar and both types of spores are readily produced.
Phyllosticta leaf spots and blights are one of the most serious problems in the commercial orchid industry. Leaf loss in dendrobium fields grown for cut sprays, and leaf rots of potted plants that are shipped, impact the quality and quantity of Hawaiis exports.
In the field or greenhouse, disease spread is due to movement of both conidia and ascospores. Conidia are splashed from older diseased leaves to other leaves nearby. Ascospores are discharged into the air and are carried greater distances. When either spore type lands on young dendrobium tissue, it germinates and forms a small structure called an appressorium. Moisture is needed for this process. The appressorium adheres to the surface of the host tissue. The fungus then grows from the appressorium and penetrates the host. Without moisture the spores fail to germinate and are unable to infect the host.
After Phyllosticta penetrates the leaf, for some time there are no symptoms or signs of the pathogen on the infected leaves. It takes 3 to 6 weeks before a small faint spot can be observed on the infected leaf. Many more weeks are required before a clearly defined yellow spot appears. Because of this, many growers are surprised to see spots developing many months following plant purchase.
In the field or in commercial greenhouses, there is sufficient sunlight for the plant to inhibit the expansion of Phyllosticta, confining the fungus to the yellow spot stage. However, when light levels are reduced the plant loses its ability to inhibit Phyllosticta. The fungus starts to grow rapidly and large rots develop. Leaves are lost and only the cane or pseudostem remains. This is a major problem for growers who ship infected potted plants. Within shipping boxes, darkness and high humidity favor disease development and plants arrive with severely diseased leaves. Similarly, light levels in garden shops are also too low to prevent disease development.
Since many orchids are propagated by seeds or cloning, it is possible to start new plants without the fungus. These plants should be protected by:
a) isolating them and keeping them away from mature plants that could have pathogens; growing new plants in a small, separate greenhouse or glasshouse would be ideal; b) preventing the movement of ascospores to new plants by keeping them in a separate greenhouse or in a section separated by solid plastic; c) reducing the number of ascospores by removing all rotted leaves and especially those in pots, and on the bench or ground.
For larger plants and for those already infected: a) remove all rotted or diseased leaves; clean benches, pots, and walkways by removing all dried sheaths, leaves, old stems etc., since the fungus survives in diseased plant material; b) trim old dead stems and flower spikes; c) apply fungicides such as mancozeb to kill fungal spores; follow label directions when using any pesticide.
Since moisture is needed for disease spread by asexual spores (conidia), for germination of the spores, and for host penetration, moisture control is very important. Potted plants grown under solid cover will be nearly free of this disease. Water plants early in the day to allow leaves to dry as opposed to just before dark, when foliage would remain wet for many hours. Remove tall bushes around fields to encourage air movement in the field and reduce long periods of high humidity.
Application of fungicides such as Dithane M45 will also reduce infection levels. However, these fungicides are not effective, once the fungus has penetrated the host. Thus, disease prevention through cultural practices and fungicide application is crucial.
1. Alfieri, S. A. Jr. 1994. Diseases and disorders of plants in Florida. Bulletin No. 14. Florida Dept. of Agr. and Cons. Serv. Div. of Plant Industry. Gainesville, Florida
2. Uchida, J. Y. and M. Aragaki. 1980. Nomenclature, pathogenicity, and conidial germination of Phyllostictina pyriformis. Plant Disease 64:786-788.
3. Uchida, J. Y. 1994. Diseases of orchids in Hawaii (Feature Article). Plant Disease 78:220-224.
COPYRIGHT: Janice Y. Uchida
Department of Plant Pathology
University of Hawaii