1. Host and symptoms

2. Parasitic cycle

3. Environment

4. Management

5. Impact

6. References

7. External links

Host and symptoms

Purple witchweed infects a variety of grasses, and legumes in sub-saharan Africa including rice, maize, millet, sugarcane, and cowpea. The symptoms mimic that of drought or nutrient-deficiency symptoms. Chlorosis, wilt, and stunting result from witchweed’s ability to extract nutrients from its host. Pre-emergence symptoms are difficult to diagnose secondary to their similarity to general lack of nutrients. Once emergence of the plant has taken place, damage has become too severe to mitigate.^[4]

Parasitic cycle

Seeds of witchweed overwinter in the soil after they are dispersed by wind, water, animals, or human machinery.^[5] When the environment is correct, and if the seed is within a few centimeters of the host root, it will begin to germinate. The germinating plant grows towards hormones, called strigolactones, released from the host root.^[6][7] The plant grows up the concentration gradient of these strigolactones. In the absence of strigolactone, the striga will not germinate. Strigolactone knockout plants have been used in an attempt to prevent infection by avoiding germination.^[7] Once in contact with the root, the witchweed produces a haustorium establishing a parasitic relationship with the plant. It remains underground for several weeks while extracting nutrients. The stem while underground is round and white. After this stage, it emerges from the ground and rapidly flowers and produces seeds. The flowers self pollinate before opening. After emergence, the plant can perform photosynthesis to augment its metabolic demands.^[6]

Environment

Witchweed’s ideal temperature for germination is 30-35°C. Below 20°C, the seeds will not germinate. Seeds can survive freezing temperatures.^[8] However, the longevity of the seed is debated. Most say that under ideal conditions, seeds can remain viable up to 14 years, but wet soils greatly decrease the resilience of the seeds. At most in one year, 74% of viable seeds were lost secondary to wet soil.^[9]

Management

Witchweed is historically among the hardest parasitic plants to control. Fusarium oxysporum may be used as a possible biocontrol of witchweed. This fungus is thought to infect the early vasculature of the striga plant.^[10] A potential solution to purple witchweed for millet and sorghum crops is herbicide priming. When herbicide-resistant seeds were soaked in herbicidal chemicals before planting, up to an 80% decrease in infestation occurred.^[11] The use of nitrogen-rich fertilizers reduces the witchweed infection rate. Although the mechanism behind this is not fully understood, the abundance of nitrogen is thought to disrupt nitrogen reductase activity. This has a ripple effect, resulting in the dysregulation of the plant's light and dark cycle, resulting in the striga's death.^[12]

In 2018, an essential protein for witchweed germination was found to consistently bind to Triton X-100 molecules, which appears to inhibit the germination of the striga seeds, preventing the natural strigolactones from binding to their usual substrate. ^[13]

Impact

In the late 1990s, "21 million hectares of cereals in Africa were estimated to be infested by S. hermonthica, leading to an estimated annual grain loss of 4.1 million tons".^[3]

References

External links

• Janthina smithiae
• Janthina umbilicata
• Janthinea
• Janthinidae
• Janthinobacterium lividum
• Janthinomyia
• Jan Thomas
• Jan Thomas Lauritzen
• Jan Thomee
• J Anthony Brown
• J. Anthony Crane
• J. Anthony Holmes
• J. Anthony Lukas Book Prize
• Jan Thorn-Prikker
• Jan Thorn Prikker (Künstler)
• Janthothorax bensbachi
• Janthothorax mirabilis
• Janti
• J Antibiot
• Jan Tille
• Jantina Hellingmolen
• Jantina Hellingmolen, Aalden
• Jantje Smit
• Jan Todd
• Jan Toeve