Striga, also known as witchweed, is a parasitic plant that is naturally found in parts of Africa, Asia, and Australia. It is classified in the family Orobanchaceae, but was previously classified in the Scrophulariaceae family. Some species of Striga are harmful to cereal crops, particularly in savanna agriculture in Africa. It also causes significant crop losses in other regions, including other tropical and subtropical crops in its native range and in the Americas. Witchweeds have bright-green stems and leaves, attractive flowers, and are hemiparasites that require a living host for germination and initial growth, but can then survive on their own. There are potentially over 40 species of Striga. Certain species of Striga, such as Striga asiatica, S. gesnerioides, and S. hermonthica, can have devastating effects on crops, especially those grown by subsistence farmers. These crops include maize, sorghum, rice, and sugarcane. The parasite can significantly reduce crop yields and cause symptoms like stunting, wilting, and chlorosis, similar to those caused by drought, nutrient deficiency, and vascular disease. Witchweed plants can produce a large number of seeds that can remain viable in the soil for more than 10 years. The seeds germinate when they come into contact with host root exudate, and then develop root cells that penetrate the host. The plant can spend several weeks underground before emerging, flowering, and producing even more seeds. The seeds are easily spread by wind, water, animals, and human activity. Striga seeds send out a root to find a host root, and if the right conditions are met, a haustorium - a structure that allows the Striga to parasitize the host - develops. The haustorium grows towards the host root, penetrates it, and forms structures called oscula that allow for nutrient transport from the host to the Striga seedling. Within 24 hours of penetrating the host, the Striga cotyledons emerge from the seed. Witchweed thrives in warm and moist conditions with temperatures between 30 and 35 C. It can also survive in frozen soil as low as -15 C. Soil type and nutrient levels do not strongly affect its growth, meaning it could potentially infect large corn crops in the American Midwest.

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