Cabbage is an important crop in Ghana where it grows all year round, right across the country. It is mainly grown for commercial production in Southern Ghana, in Akwapim and Kwahu areas and in the moist high elevations around Tarkwa.
Black rot is considered the most important disease of crucifers across the world and can attack its host at any stage of growth. Cauliflower and cabbage are the most readily affected crucifer hosts and suffer significant yield loss as a result of the disease. On cabbage, black rot causes yellow to brown V-shaped lesions to develop on the edges of leaves and move inwards towards the midrib. As the disease progresses, the lesions turn darker, and leaves may wilt and fall from the plant. In the advanced stages of the disease, veins in the affected area will darken.
The disease is causes by a bacteria, Xanthomonas campestris pv. campestris, which can be spread via wild hosts, water or, most commonly, infected seed. Even symptomsless plants may produce infected seed so it is important to try to source certified disease-free seed before planting. For more information about how to control this disease, read September’s Factsheet of the Month, Control of Black Rot in Cabbage, which has been written by staff from the Ministry of Agriculture in Grenada.
U.S. Department of Agriculture (USDA) scientists in Illinois, USA, are investigating the role of anthocyanins in pest-control. They believe that the plant pigment can adversely affect crop pests such as the corn earworm caterpillar and the cabbage looper caterpillar that feed on it.
Anthocyanins are a plant pigment which give blackcurrants and flowers, such as petunias, their blue and purple colour. They absorb blue-green and UV light, protecting plant cells from high-light stress.
In the experiments the scientists used corn earworm caterpillars (Helicoverpa zea). H.zea damage is usually serious and costly because of the larval feeding preference for the reproductive structures and growing points which are rich in nitrogen.
The caterpillars were forced to feed on blue areas of petunia petals which contained higher levels of anthocyanins than the white areas. It was found that these individuals gained less weight than other individuals which were fed on only the white leaf areas. Further experiments found that isolated anthocyanins slowed the caterpillar’s growth rate.