Study on epidemiology of lavender root rot and its non-pesticide control

Lavandula (common name lavender) is a flowering and aromatic shrub in the mint family, Lamiaceae. Lavender was derived from temperate area such as the Mediterranean countries, and mainly distributed over France, Spain and Italy. It prefers the dry and cool weather. Since lavender introduced from abroad, it was observed that the hot and humid environment in summer in Taiwan is not suitable for lavender to grow. It is thought that the hot and humid environment make lavender not easy to overcome the summer. This study and previous reports, however, show that lavender is frequently infected by Phytophthora nicotianae, and the root rot disease is the key problem in summer. This disease can cause the browning root rot , the discolored leaves and plant wilt. The severely infected plants will die. This study is also aimed to identify whether the environmental condition influence the occurrence of lavender root rot. Furthermore, several non-pesticide material, such as phosphorus acid and herbal plant extracts are also treated for their role in controlling the root rot disease. Through the pathogenicity test and molecular identification, we accomplish the Koch’s postulates of the disease caused by Phytophthora nicotianae. Culturing the pathogen at different temperature showed that the pathogen grows best at 22℃, although the myceliumcan grow at temperature range from 16℃ to 37℃. The study of lavender root rot disease at different temperature showed that the pathogen could infect the host lavender at 20℃ and the disease severity increased at high temperature of 35℃. The non-inoculated lavender still keep healthy at 40℃ for two weeks. It shows that the wilt and death of lavender in summer is caused by root rot disease rather than high temperature. The study of lavender root rot disease at different water potential showed that the three levels of water potential make no significant difference to disease incidence and severity, although the non-inoculated lavender appeared a slight drought injury in low water potential. The study of lavender root rot disease in flooding situation showed that the disease incidence and severity in flooding is less serious than non-flooding in three days, but it make no difference between both three days and five days. The non-pesticide control study showed that five plant extracts could inhibit the zoospore germination and mycelial growth significantly, but they didn’t have good effectiveness in the pot plant test. In the plot plant test, the neutralized phosphorous acid solution(NPA) and phosphoric acid had better effectiveness than the ethanol extract of clove and galla rhois. No matter pre-inoculated or post-inoculated, the sprayed NPA showed good effectiveness in reducing the disease. The NPA at 500X reduced the disease severity to 11% in pot plant test aftere pre-applied at seven days prior to inoculation, and reduced to 22% in pot plant test when applied after inoculation. The pre-application of phosphoric acid at 500X reduced the severity to 6% when applied before inoculation. Furthermore, the application of phosphoric acid is easier than NPA, and can not be affected by environment factors. Nine fungicides were treated for their inhibition effects on the pathogen. Among them, Metalaxyl, dimethomorph, kasugamycin＋copper oxychloride, fosetyl-aluminium and copper hydroxide + oxine-copper showed good inhibition in the mycelial growth test. The application of metalaxyl and dimethomorph recommended for controling the disease. However, it is reported that both has fungicide resistance problems. Therefore, they should be rotated with different fungicide of different mechanisms of action.