Alternaria Leaf Spot might be popping up on your broccoli, Brussels sprout, cauliflower and other members of the Brassica family as days become cool at the end of summer. That is according to the Extension Service of the University of Massachusetts.

I recently read their Vegetable Notes newsletter from August 2024, and it said Alternaria Leaf Spot, a common disease of brassicas especially in the Northeast, is something to keep your eye out for during the seasonal turnover. Its growth is more prevalent in cooler temperatures and during extended periods of leaves being wet. We’ve had a moist summer so far—moist both from rain, dew and humidity—and that is likely to continue through fall. Which means, heads up for this disease to spread in the coming months.

What Exactly is Alternaria Leaf Spot?

Brassicas can be impacted by a bunch of different species in the fungal Alternaria genus, but it’s the ones that favor brassicas specifically—A. brassicae and A. brassicicola—that can do the most damage to your cole crops.

How do you know you have Alternaria Leaf Spot? They appear as small black dots on leaves that are encircled by crusty light-brown rings. As these spots grow, they become lesions round spots with concentric rings that look like a target. Also as they enlarge, the spots meld into one another, creating large dead spots in the leaves, and the leaves may eventually drop on their own. These lesions appear on older leaves first. And it’s not just leaves! These spots can also appear on the floret heads of broccoli and cauliflower.

How It Spreads

The method of infection is via its fungal spores. Spores can persist from year to year by overwintering on debris left on the ground in your garden. Alternaria spores get spread numerous ways. These include:

• Traveling via wind
• On seed
• Via water in fields
• Via flea beetles

Methods to Control Alternaria

To prevent and control the spread of Alternaria Leaf Spot, you have to address its spores. The best thing to do is to collect and remove from your property any plant material that exhibits spores and disease. Don’t compost it, just remove it completely. Other methods of control are:

• Refrain from planting successive brassica crops (spring then fall crops) near each other; i.e. keep them separated to avoid contamination.
• Keep weeds to a minimum, to avoid giving spores an alternative host plant.
• Do not water from overhead. You can’t prevent rain but try your best to irrigate your garden (brassicas and all) from the base.
• Purchase disease-tolerant varieties of brassicas. There are a few varieties available. A quick Google search says Emerald Jewel, Green Magic and Marathon are varieties to look for. Read more on that HERE.
• Flea beetles can spread Alternaria, so controlling flea beetles will help stem the disease’s spread.
• If you are planting brassica seeds, buy certified, disease-free seed.

It seems that the best approach to controlling Alternaria and preventing its spread is to “garden smart;” i.e. do all the things that help to control pretty much all disease. I know time can be short and efforts can wane, especially near the end of the season. But consistent effort in tamping down diseases will pay off with a disease-free harvest at the end of the season.