(1) Field of the Invention
The instant invention is a method for the control of undesirable plants by use of plant pathogens.
(2) Description of the Prior Art
The merits for using plant pathogens to control weeds in annual crops have been discussed previously for two Colletotrichum spp. (Daniel, et al. U.S. Pat. No. 3,999,973). The anthracnose fungus Colletotrichum gloeosporioides has been used to control the weed northern jointvetch, and another strain of this fungus has been used to control winged waterprimrose. Colletotrichum malvarum has been used to control prickly sida. These three pathogens have been combined to control all three target weeds at once. In other experimental work the fungus Alternaria macrospora has been used to control spurred anoda, Alternaria macrospora, Weed Science, L. Walker, 1981, Vol 29, pp 505-507.
A major constraint to commercial development of a plant pathogen as biological herbicide is selectivity. A pathogen that controls only one weed species in one crop does not have the same market potential as a pathogen that controls several important weeds in several crops.
Sicklepod (Cassia obtusifolia L.) is a major weed problem in much of the southern United States where soybeans and peanuts are grown. This non-nodulating legume is very competitive with these crop plants and can significantly reduce yields at low weed densities. Mature sicklepod plants commonly reach a height of 2 to 2.5 m. The weed produces large quantities of seeds that can germinate and grow under a wide range of environmental conditions. Seedlings characteristically have rounded cotyledons, 15-20 mm across, with 3 to 5 distinct veins in the upper leaf surface. The first leaves have 3-5 leaflets that are rounded at the tip.
Sicklepod control with herbicides is difficult. An emergency use permit has recently been issued by the United States Environmental Protection Agency to allow the use of toxaphene for sicklepod control in soybeans grown in 5 southern states. Also, metribuzin can be applied postmergence directed, but this chemical is sometimes injurious to the soybeans.
Coffee senna (Cassia occidentalis L.) is similar in appearance to sicklepod, except that the seed pods are shorter, straighter, and more flattened. This species is widely distributed throughout the southeastern United States, and is an important weed within much of its geographic range.
Showy crotalaria (Crotalaria spectabilis Roth.) is widely distributed in the southeastern United States. This species is a problem weed because of the poisonous seeds that it produces. These seeds are very toxic to livestock, and a major source of poisoning is from the consumption of grain or feeds contaminated with crotalaria seeds. This species is also a member of the family Leguminosae.
A. cassiae is the first fungal pathogen that has been used to control these weeds.