Current approaches to combating B. lactucae are based on genetic resistance and fungicides. Controlling the disease in organic production is dependent on a small number of approved products, including copper hydroxide. However, use of this heavy metal may have long-term consequences due to its accumulation in soil, furthermore copper fungicides also cause phytotoxicity. In conventional production, the disease can be controlled by synthetic chemicals, including protective and systemic fungicides such as phenylamide (e.g., metalaxyl/mefenoxam-based compounds, and phosphites). However, isolates that are insensitive to these compounds are common.
The natural Australian tea tree oil derived from the Melaleuca alternifolia plant contains many components, mostly terpenes (p-cymene, terpinen-4-ol, terpinolene, 1,8-cineole, α-pinene, α-terpinene, γ-terpinene), sesquiterpenes, and their respective alcohol (monoterpene alcohol-terpineol). It was found effective against a broad range of plant-pathogenic fungi in numerous crops.
"We have evaluated the antifungal activity of essential Australian tea tree oil (TTO) against downy mildew in lettuce. Preventive treatments with Timorex Gold (STK Ltd., Petach Tikva, Israel), fungicide containing TTO, suppressed the development of B. lactucae on cotyledons and young lettuce plants - the scientists at University of Haifa refer - Epifluorescence microscopy showed that TTO had a moderate inhibitory effect on spore germination of the pathogen but a strong inhibitory effect on sporophore emergence and hence on sporulation. Timorex Gold was as effective as copper hydroxide in controlling the disease in organic field plots. It was as effective as the fungicides Bellis (Boscalid + Pyraclostrobin; BASF, Germany) and Commet (pyraclostrobin; BASF, Germany) in conventional production. The results demonstrate that Timorex Gold effectively suppresses the development of downy mildew in lettuce in growth chambers as well as in the field, and thus suitable biopesticide for its control."
Source: Moshe Reuveni and Yigal R. Cohen, 'Essential Tea Tree Oil Activity against Bremia lactucae in Lettuce', 2020, Agronomy, 10(6), 836.