Some of my broccoli seedlings are developing spots on the leaves. They are mostly a black spot and gray on the bottom of the leaf. This problem is affecting the bottom most leaves first and sort of working its way up the plant. This problem is affecting only some of the beds with broccoli planted in and not other beds.
I can’t treat the problem until I can correctly diagnosis it.
This is the bottom of a broccoli leaf. The leaf in the top right corner is a basil leaf. The spots look different – probably another problem.
Looking online, I found 2 possibilities: Downy Mildew – and alternaria. Actually, it doesn’t resemble absolutely either issue, aka black leaf spot and gray leaf spot.
Downy Mildew – Peronospora parasitica
- Gray-white sunken leaf spots which are often angular and restricted by the leaf veins
- Leaf tissue around the spots turns yellow
- Fuzzy gray growth can be seen on the underside of the leaves
- Numerous black sunken spots can form on infected heads
- Disease is common in cool wet weather
Black Leaf Spot/Gray Leaf Spot – Alternaria spp.
- Gray to black round leaf spots with concentric rings
- Leaf tissue becomes dry, brittle and often falls out, resulting in a ‘shot hole’ appearance
- Leaf spots often appear first on lower older leaves
Neither of these symptoms absolutely accurately describes the problem, so I don’t know what to think. At this point, I can only hope that the issue stops racing up my broccoli plants. We had several days of wet weather, but are going to be dry for a few weeks – hopefully this will help.
Update- I did a bit of online research and decided to spray the plants in question. I added 4 tsp of baking soda and 1 ounce of Need oil to a gallon of water (but only used half of it – put the other half gallon in a jug -hopefully it will keep for a week or so in case I need to spray again). I sprayed the broccoli and cabbage plants with yellowing leaves. I sprayed mostly the bottom of the leaves, but of course lots of the spray also got on the top of leaves. I am a bit concerned about spraying baking soda on plants – previous experience has showed me that squash plants do not like this solution on their leaves. Will update and let you know how this spray worked on the broccoli leaf spots.
Update Update- as of early December, these plants are under a plastic hoop cover. There is very little trace of yellowing or spotted leaves, but the plants seem stunted. Other broccoli & cabbages planted outside in the old tomato bed have grown taller than these, as have a few brocs in an adjacent covered bed. It could be that two treatments killed off what I think was downy mildew, or could be the cold weather – we had to very cold blasts early in November – but for whatever reason, the surviving plants don’t seem to be growing as they should – in fact, they don’t look like they have gotten any bigger in the past month! Also, this is the same half of the garden where my yellow wax beans that I planted in late summer did not thrive – DowAgriscience’s environmental toxin Aminopyralid revisited??