Invasive Flora with Symbiotic Characteristics
Parasitic plants, which derive nutrients from their host organisms, can have a significant impact on crops and trees. Three notable examples are Cuscuta spp. (dodders), Cassytha spp., and Hydnora africana.
Dodders (Cuscuta spp.) are obligate parasitic plants that affect a broad range of hosts, including important crops such as alfalfa, tomato, sugar beet, and legumes. They reduce host vigor, suppress growth, drastically decrease yield, and interfere with nutrient flow and photosynthesis. Additionally, Cuscuta can serve as a vector for plant viruses and other pathogens, worsening crop health. Their broad host range and rapid growth make them highly invasive agricultural pests.
Cassytha spp., parasitic vines resembling dodders, can also damage crops and trees by parasitizing them. While generally considered less serious pests than Cuscuta, some Cassytha species can stunt growth and sometimes kill affected plants.
Hydnora africana, a parasitic flowering plant native to southern Africa, lacks leaves and chlorophyll, living underground by parasitizing roots of plants, especially Euphorbia species. Its bizarre flower emerges above ground and uses a strong odor to attract insects for pollination. While primarily known from wild ecosystems, its parasitism of roots can severely affect host plants.
These parasitic plants reduce crop yields by impairing host photosynthesis and nutrient uptake. They can stunt growth and sometimes kill affected plants. Cuscuta notably spreads plant viruses between infected and healthy hosts, amplifying crop loss. Parasitic root feeders like Hydnora affect the root systems of their hosts, potentially weakening trees and other plants. In agriculture, infestations of dodders on legumes, tomatoes, sugar beets, and other crops cause significant economic losses due to reduced vigor and yield.
Mistletoe, another member of the hemiparasitic group, attacks many broad-leaved trees and some conifers. It germinates on limbs of susceptible hosts, forms an attachment disk on the bark, and is disseminated throughout the tree and from tree to tree by birds and wind. The usual control is to cut out the mistletoe branches deep into the tree proximal to the point of attachment, but no good herbicidal control has been developed. The best control is to plant only tree species resistant to mistletoe attacks.
Strict regulations prohibit the sale of crop seed contaminated by dodder seed. In cases of infestation, patches of dodder in field crops or along fences or ditch banks should be eradicated by burning or by herbicides. Witchweed, a hemiparasitic seed plant, damages a wide range of crops, including sugarcane, corn, sorghum, and many other grasses and broad-leaved plants. The best control is to plant a crop that stimulates the witchweed seed to germinate, then plow under the entire field, rotate crops, and avoid planting susceptible crops.
In summary, true parasitic plants like Cuscuta, Cassytha, and Hydnora africana have substantial negative effects on a wide range of crops and trees by depriving them of nutrients, spreading diseases, and impairing their growth and productivity. Careful attention should be paid to planting resistant species, eradicating infestations, and regulating the sale of contaminated seed to minimise their impact.
