A couple of weeks ago I started to notice some bronze type patches on my basil:
|Bronze Patches on Sweet Basil Leaves|
I didn't think too much of it at first. I assumed it was a bit of sunburn. And even though basil is a sun loving plant, I had read that they can become sunburned if they get too little water and too much sun. I'm not the most conscientious when it comes to standing their, spraying a bed, so I bumped up the watering a bit.
But the basil didn't get better. In fact it got worse with each passing day and dark brown patches began to appear.
|Basil with Bronze Patches and Brown Spots|
At first, only the sweet basil was affected, but in the last few days, I began seeing signs of the same thing on the Thai basil. Yesterday I turned over a leaf and that is when I realized that this was definitely not a simple case of too much sun & not enough water. Off to the computer I went and, after a bit of research, I discovered that it was Basil Downy Mildew.
|Basil Downy Mildew Spores on Undersides of Leaves|
I have never encountered downy mildew in the garden before, so it was a bit of a surprise. And I was even more surprised when I discovered that there is a form of downy mildew that is specific to basil.
Basil Downy Mildew is caused by a parasitic organism - Peronospora belbahrii - yeah, I can't pronounce it either. It thrives in warm, humid conditions - which basically describes the typical Southern Ontario summer. And apparently there is a bit of an outbreak this year.
The spores are usually carried through both infected seeds & wind. I have no idea how my plants became infected - it could have been either of these sources. If it was through the seed, that means that at least one of the three packets I sowed was infected. I'm pretty confident that the Thai basil seed is fine as it was the last to show any signs of infection. I am going to toss both Genovese basil packets, just in case, and purchase fresh sweet basil seed next spring. Downy mildew does not survive winters in the north, so next year, I will still be able to plant basil in my herb bed.
If you want to find out more about Basil Downy Mildew, there is a very extensive article on the Cornell University Vegetable MD website that was just posted in February 2014.
All of my plants were pretty much goners, so I have pulled them up.
|Genovese Basil Infected with Downy Mildew|
- Just before I pulled them all up -
|Thai Basil - Also Infected & Pulled Up|
The pictures actually make them look a lot better than they were. Practically every leaf had mildew spores underneath.
The best way to avoid or minimize future episodes is to keep the plants dry. Ideally, they should be planted in a nice sunny spot with good air flow and drip irrigation. The first two conditions were present in my garden. But, unfortunately, I don't have drip irrigation installed in the herb bed yet, so my overhead watering likely made the infection progress significantly faster than it otherwise would have. There is little that can be done once the mildew is evident on the plants. I don't use fungicides, but even if I did, they are ineffective unless you use them before the plants show signs of infection (the Cornell article has quite a detailed section on the use of fungicides).
I'm not yet sure what I am going to do with the newly vacated section of the herb bed - I may sow some more dill and/or cilantro. But even though my basil is done for the year, I'm thankful that I was able to get in a few good harvests before the mildew took over.
This post was shared on Green Thumb Thursdays.
Till next time...☺