Basil downy mildew is one of the most destructive diseases affecting basil plants. This fungal disease can quickly defoliate and kill basil plants, rendering them unusable for culinary purposes. However, with proper prevention and control measures, basil downy mildew can be successfully managed. This comprehensive guide provides useful tips and techniques for dealing with basil downy mildew to help grow your own healthy, productive basil plants.
What Causes Basil Downy Mildew?
Basil downy mildew is caused by the fungal pathogen Peronospora belbahrii. This oomycete fungus spreads via tiny spores called sporangia that travel by wind, water, and infected seed. The sporangia germinate on the underside of basil leaves, sending threadlike hyphae into the leaf tissue that extract nutrients and release toxins, causing the leaves to yellow and die. Cool, wet conditions provide optimal conditions for basil downy mildew infection and spread.
Recognizing Basil Downy Mildew Symptoms
Identifying basil downy mildew early is crucial to control the disease before major damage occurs. Here are the key symptoms to look for:
- Yellowing of leaf edges or entire leaf surface
- Leaf curling and crinkling
- Grayish fuzzy sporulation on the underside of leaves
- Dark brown necrosis developing on leaf tips and edges
- Rapid leaf drop starting from the bottom of the plant
- Stunted growth and lack of vigor
If you spot these signs, especially the gray fuzzy sporulation, be sure to act quickly before the fungus spreads further.
Preventing Basil Downy Mildew Infection
An ounce of prevention is truly worth a pound of cure when it comes to managing basil downy mildew. Here are some useful preventative measures:
Choose Downy Mildew Resistant Varieties
Look for sweet basil cultivars described as “DMR” which indicates they have resistance to downy mildew. Good options include Astra, Eleonora, Poppy Joe’s, Rutgers Devotion DMR, Thunderstruck DMR and African Blue DMR. Planting resistant varieties can greatly reduce susceptibility.
Provide Proper Sun Exposure
Basil thrives in full sun. Allowing at least 6 hours of direct sunlight daily reduces moisture on the leaves that the downy mildew fungus needs to infect plants. Proper sun exposure also strengthens plant defenses.
Improve Air Circulation
Good air movement through and around basil plants inhibits downy mildew spore germination and spread. Use row covers and trellising to prevent overcrowding. prune plants to open up the canopy. Space plants 1-2 feet apart for optimal air flow. Set up oscillating fans in the basil growing area to keep air circulating.
Water Early in the Day
Water basil at the base in the early morning hours. This allows the leaves to dry out during the day, denying downy mildew the moister conditions it prefers. Avoid overhead watering and late day watering.
Disinfect Tools & Equipment
Clean and disinfect pruning sheers and other garden tools with diluted bleach, Lysol or other disinfectants between plants to avoid spreading downy mildew spores. Also disinfect greenhouse surfaces, stakes, ties and any other equipment used in the basil growing area.
Scout for Symptoms
Inspect basil plants frequently, at least twice a week. Look for early signs of yellowing leaves and isolate immediately any plants showing symptoms to avoid spore spread.
Remove volunteer basil plants
Self-seeded basil plants can harbor downy mildew. Remove any volunteers to eliminate potential reservoirs of downy mildew inoculum.
How to Treat & Manage Basil Downy Mildew
If preventive measures fail and you find basil downy mildew infections in your crop, take action right away with these treatment and management strategies:
Remove & Destroy Infected Plants
Uproot and discard any basil plants showing downy mildew symptoms. Put them in sealed bags and dispose of them far away from the growing area so fungal spores do not spread. Sterilize tools after removal. Removing infections is critical to control spread.
Apply Fungicides
Certain fungicides are effective for combating basil downy mildew when applied early at the first onset of symptoms. Useful options include phosphonic acids, Mandipropamid, mono and dithiocarbamates and chlorothalonil. Always follow label directions. Rotate between fungicide classes to avoid resistance.
Increase Air Circulation
Improving air flow is even more critical once plants start showing infection. Position oscillating fans nearby, trellis or stake plants to open up spacing. Prune lower leaves and interior branches to support better penetration of sunlight and air movement within the plant canopy.
Alter Watering Practices
Once downy mildew is identified, water early in the day at soil level and avoid overhead irrigation. Remove any branches or plants touching the ground to reduce spread of spores kicked up from soil onto foliage by water splash.
Apply Biological Controls
Some beneficial microbes can help protect against foliar pathogens like downy mildew. Applying sprays of Bacillus subtilis, Bacillus amyloliquefaciens and Streptomyces lydicus can protect leaves and crowd out fungal growth. Always follow label instructions.
Use Row Covers
Installing floating row covers over basil plants can provide a physical barrier against wind and rain splash dispersal of downy mildew spores. Just be sure to allow sufficient ventilation during the hottest part of the day.
Grow basil indoors
If downy mildew persists and outdoor conditions are highly favorable to the disease, consider moving some basil plants indoors under grow lights or in a sunny window. This can provide a protected environment and fresh harvest source.
Scout other plants
Downy mildew fungi can sometimes overwinter on related plants. Scout for symptoms on cole crops, roses and other species. Remove any identified hosts promptly.
With diligence and prompt action at the first sign of infection, basil downy mildew can be successfully mitigated. Consistent prevention and control measures allow for ongoing harvest of healthy and usable basil.
Frequently Asked Questions About Basil Downy Mildew
Many gardeners have questions when dealing with the challenges posed by basil downy mildew. Here are answers to some of the most common queries.
Is basil downy mildew edible?
No, basil infected with downy mildew should never be consumed or used for culinary purposes. The fungal disease renders the basil unusable for eating. Only harvest and use uninfected, healthy basil plants.
What temperature kills basil downy mildew?
While extended periods below 40 F can temporarily suppress basil downy mildew, sporangia and oospores of the fungus can survive down to 23 F. For complete control, maintain greenhouse temperatures at 104-122 F for at least 4 hours.
Can infected basil plants recover?
Unfortunately basil plants infected with downy mildew cannot recover. The systemic infection and defoliation is too severe. Remove and destroy infected plants promptly before the disease spreads. Healthy plants can be protected from infection using preventive practices.
Does neem oil prevent basil downy mildew?
Applying neem oil to basil plants can deter downy mildew infection but is more effective as a preventive measure, not after infection occurs. The azadirachtin in neem oil possesses antifungal activity against downy mildew.
How long can basil downy mildew spores live?
Under cool, damp conditions, basil downy mildew sporangia can survive up to 15 days, allowing ample time for the spores to infect healthy plants. Sanitation and removing sources of inoculum are critical in controlling this persistent disease.
Can you plant basil again after downy mildew?
It is best not to replant basil in a growing area that experienced downy mildew outbreaks the same season. Wait until the following year after ensuring the fungus has nothing to overwinter on. Be sure to sterilize any reused pots, trays and tools before planting new basil.
Does composting infected plants spread downy mildew?
Composting can destroy many pathogens, but some downy mildew fungi may persist. It is safest to bag infected plants and dispose of them in the trash rather than the compost pile to prevent spreading downy mildew inoculum back into your garden soil.
Conclusion
Basil downy mildew can be a frustrating challenge for gardeners and basil lovers. However, by understanding what causes the disease, learning to recognize symptoms early and employing smart preventive practices, outbreaks can be avoided. Removing infections promptly, improving plant care and applying treatments at first onset of disease can help manage basil downy mildew if it does strike. With proper integrated pest management, a flourishing and bountiful basil harvest is still possible even in the presence of this destructive pathogen.