This is the ninth fact sheet in a series of ten designed to provide an overview of key concepts in plant pathology. Plant pathology is the study of plant disease including the reasons why plants get sick and how to control or manage healthy plants.
Some higher plant forms live on the surface of or parasitize other plants and often cause harmful reactions in their hosts. These plants can be placed in three groups: the epiphytes, the hemiparasites, and the true parasites.
|Leafy mistletoe on an oak tree. Photo courtesy F. H. Tainter, © The American Phytopathological Society|
Epiphytes do little or no harm to their host plants, using them merely for physical support and protection. Examples are Spanish moss and epiphytic orchids, which in their native habitats commonly grow on tree limbs.
Hemiparasites, sometimes called water parasites, do injure their host plants, absorbing water and mineral nutrients from them. However, they possess chlorophyll and can manufacture their own carbohydrates by photosynthesis.
Witchweed (Striga asiatica) is a hemiparasitic seed plant that severely damages sugarcane, corn, sorghum, many other grasses, and some broad-leaved plants. It attaches itself to the host’s roots and utilizes most of the host’s water and mineral nutrients, causing it to wilt, yellow, grow poorly and die. The best control is to plant a crop, such as Sudan grass, that stimulates the witchweed seed to germinate, then plow under the entire field. Crops should be rotated and susceptible crops should not be planted.
Mistletoe (Phoradendron spp.), another member of the hemiparasitic group, attacks many broad-leaved trees such as ash, silver maple, honeylocust, hackberry, cottonwood, walnut, oak, birch, sycamore and some conifers. The seeds germinate on limbs of susceptible hosts, forming an attachment disk on the bark. The sticky berries are disseminated throughout the tree and from tree to tree by birds and wind. The usual control, although not very effective, is to cut out the mistletoe branches deep into the tree proximal to the point of attachment. No good herbicidal control has been developed, although a dormant-season application of ethephon, an ethylene-releasing material, is a possible control on some plants. The best control is to plant only tree species resistant to mistletoe attacks.
True parasites lack chlorophyll and depend upon their hosts for all nourishment—carbohydrates, minerals and water. Examples of this group are the dwarf mistletoe (Arceuthobium spp.) and dodder (Cuscuta spp.). Broomrape (Orobanche spp.) is a serious parasitic pest in Europe, Asia and Africa and has caused extensive damage to tomatoes in California.
Dwarf mistletoe attacks many coniferous species in the western United States, reducing tree vigor and lowering lumber quality. The sticky seeds (not the fruits) are forcibly ejected and can travel up to about 19 meters (60 feet). This is the principal means of dissemination. Birds are known to carry the sticky seeds on their feathers, but wind plays a very minimal role in dissemination. The best control is removal of infected trees.
Dodder has many species, but about six cause the major damage, attacking such crops as alfalfa, lespedeza, clover, flax, sugar beets, potatoes and some vegetable crops as well as ornamentals. Dodder seriously reduces yields and quality of crops. Strict regulations prohibit the sale of crop seed contaminated by dodder seed. Great effort should be taken to avoid planting seed that has dodder seed mixed with it. Patches of dodder in field crops or along fences or ditch banks should be eradicated by burning or by herbicides.
This is the ninth fact sheet in a series of 10 designed to provide an overview of key concepts in plant pathology. Plant pathology is the study of plant disease including the reasons why plants get sick and how to control or manage healthy plants.
For detailed information on each of the IPM strategies, see the fourth fact sheet in this series, “Keeping Plants Healthy: An Overview of Integrated Plant Health Management” (PLPATH-GEN-4).
Introduction to Plant Disease Series
- Plants Get Sick Too! An Introduction to Plant Diseases
- Diagnosing Sick Plants
- 20 Questions on Plant Diagnosis
- Keeping Plants Healthy: An Overview of Integrated Plant Health Management
- Viral Diseases of Plants
- Bacterial Diseases of Plants
- Fungal and Fungal-like Diseases of Plants
- Nematode Diseases of Plants
- Parasitic Higher Plants
- Sanitation and Phytosanitation (SPS): The Importance of SPS in Global Movement of Plant Materials
These fact sheets can be found at OSU Extension’s Ohioline website: ohioline.osu.edu. Search for “Plant Disease Series” to find these and other plant pathology fact sheets.