Management of Crown Gall Disease in Selected Rosa Hybrida Farms in Kenya Using Artemisia Annua Leaves and Zingiber Officinale Rhizome Extracts
Njagi, Alfred Kariuki
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ose flower is the world’s most traded cut-flower with 74% of it coming from Kenya. Kenya has potential for higher rose production if challenges of pests and diseases are dealt with. Pests like spidermites, false codling moth, caterpillars, aphids, thrips, nematodes and diseases such as crown gall, downy mildew, powdery mildew and botrytis highly compromise production. Crown gall disease, caused by Agrobacterium tumefaciens is the most problematic disease of roses in Kenya and causes a production loss of up to 60% depending on age and variety. This study evaluated the prevalence of crown gall disease and management of the disease using Zingiber officinale rhizome and Artemisia annua leaves extracts, in an effort to replace conventional chemicals due to their environmental and economic cost. Survey of selected flower farms in Kenyan showed a crown gall prevalence ranging from 0.1% to 65%. Zingiber officinale and Artemisia annua phytochemical screening indicated presence of terpenoids, flavonoids, alkaloids, saponins, glycosides, phenols and tannins. Agrobacterium tumefaciens colonies used in testing antibacterial effect of the extracts were isolated from crushed crown galls and purified through their growth in Congo red YEMA medium and yeast peptone glucose agar. The colonies were authenticated biochemically by Gram staining, motility test, citrate utilization, catalase production, urease production and ketolactose test. Antibacterial effect of the extracts were determined by measuring the diameter of the inhibitory zone around the filter discs soaked in extracts on Agrobacterium tumefaciens inoculated media. Micro-dilution technique on microtiter plate was used to determine the minimum inhibitory concentrations of the plant extracts used in soaking the filter discs. Artemisia annua, Zingiber officinale, mixture of Zingiber officinale and Artemisia annua had Minimum inhibitory concentration of 125mg/ml, 62.5mg/ml and 31.25mg/ml respectively. The recommended rate (6.25 ml/l) of copper hydroxide was used. Copper hydroxide and Artemisia annua had antibacterial inhibitory zone of 12.8mm. This zone was significantly different from that of Zingiber officinale and mixture of Zingiber officinale and Artemisia annua which was 10.6 and 10.2 mm respectively (P ≤0.05). The research therefore revealed that Artemisia annua and copper hydroxide inhibited bacterial growth better than Zingiber officinale and mixture of Zingiber officinale and Artemisia annua. Results of treatments done on inoculated rose plants in the greenhouse in terms number of plants with galls, gall weight and stem length showed similar tread to in vitro bacterial growth inhibition. The ultimate stem length of Artemisia annua and copper hydroxide treatment were 69.4cm and 65.8cm respectively. These lengths were better and significantly different from that of Zingiber officinale and mixture of Artemisia annua and Zingiber officinale which were 48.8cm and 54.4cm respectively (P ≤0.05). From the results of this research, Artemisia annua and Zingiber officinale extracts are promising biocontrols for crown gall in roses. Farmers are recommended to continue with integrated crown gall control methods and pursue plant extracts as an alternative. The performance of Artemisia annua extracts compared well with that of conventional copper hydroxide and is therefore the better option.