Inhibitory efficacy of several plant extracts and plant products on Phytophthora infestans

The effects of garlic and scouring rush extracts and two plant products Bio clean and Citronella Oil, on spore germination, hyphae growth and infection of Phytophthora infestans (Mont.) de Bary on potato leaves were evaluated and measured in comparison with protective fungicide chlorothalonil and distilled water which was used as control. Highest inhibitions on germination of P. infestans were obtained with Bio clean and garlic (Allium sativum L.). The inhibition rates on germination of sporangia and zoospores increased with increasing concentrations of these extracts and reached as high as 100 percent when the dosages were high. The germination of zoospores was totally stopped by all the concentrations of Bio clean and garlic. Scouring rush also showed antifungal properties but with less extent. Citronella oil didn't show effectiveness on inhibiting sporangium germination but reduced the germination rate of zoospores. Garlic extracts from fresh bulbs had stronger inhibiting effect than the extract from dry garlic bulbs on the germination of sporangia and zoospores. However, there was no significant difference between the extracts of immediately made from fresh bulbs and those which was kept for a week under 4℃. The effect of scouring rush extract from fresh leaves was not significantly superior over the extract from dry leaves and the extract from fresh leaves but stored for one week on inhibiting sporangium germination. However, the inhibiting effects against zoospore germination were significantly different between the extracts from fresh leaves and that from dry leaves. Tests were made on inhibiting effects of plant extracts and plant products on hyphae growth of P. infestans on petri dishes. No mater autoclaved or not, Bio clean 0.125% totally inhibited the formations of P. infestans colonies, which indicated that Bio clean is not sensitive to heat. The inhibiting effect of garlic was as good as Bio clean when it was not autoclaved. However, its effect lost completely when it was autoclaved. The inhibitory compound was possibly thermolabile. In general, the filter sterilized extracts had greater inhibitory effect on the fungus than the autoclaved samples. The effectiveness of various extracts on hyphae growth was in the decreasing order of Bio clean, garlic, scouring rush and Citronella oil. All the plant extracts and plant products had no curative effects on development of P. infestans on potato leaves. Bio clean (0.125%) had significant influence on inhibiting the infection of P. infestans when it was used at the same time to three days earlier than inoculation. Its effect was as good as chlorothalonil (0.1%). Garlic (2%) had strong inhibiting effect when it was used one day earlier or at the same time of inoculation. The inhibition rate reached 89% and 100% respectively. Comparing to untreated check, souring rush (4%) and Citronella oil (400 mg·kg -1 ) had some effects when they were used 1～2 days earlier than inoculation. However, the disease prevention effects were not enough. These results showed that Bio clean and garlic have strong antifungal properties and there are great potential for using them in potato plant protection against P. infestans .