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Alternaria Leafspot and Head Rot on Broccoli

Posted: October 19, 2015

In an experiment at the Russel E. Larson Agricultural Research Farm we have alternaria leafspot and head rot on the leaves and heads of fall broccoli planted. It’s a common late-season disease.
A healthy broccoli leaf (on the left) next to one with alternaria leafspot (on the right).

A healthy broccoli leaf (on the left) next to one with alternaria leafspot (on the right).

When we visited Brian Campbell Farms last year, Rachel Troyer, the food safety and integrated pest management manager, mentioned it as one of the major diseases that is very closely monitored on the farm’s 96 acres of broccoli.

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Alternaria head rot on broccoli. Notice the yellow/brown/black spots that make the heads unsaleable.

Alternaria leafspot and head rot on broccoli is commonly caused by a couple of fungi: Alternaria brassicicola and Alternaria brassicae. Disease development is favored by cool temperatures and long periods (more than 9 hours) of high moisture. Fungal spores can be moved in a variety of ways: with wind currents, with splashing water, on equipment, on animals—including people, and in/on infected seed.

Leafspot symptoms start off as small yellow spots on older leaves that develop into ½ inch diameter or larger “bull’s eyes” or “targets” with concentric rings of varying shades of tan/gray/black. Alternaria head rot also starts off as yellow spots that turn brown and black. You will see black spores on spots on leaves or heads.

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Note the spots of concentric rings of varying shades of tan/gray and black which is characteristic of alternaria leafspot.

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Close-up of a lesion of an alternaria head rot spot.

Alternaria head rot can be confused with bacterial head rot.  Bacterial head rot starts off as water-soaked spots on the head. Spots are soft and have a very bad odor as the disease develops.

There are some things you can do to minimize or avoid having this alternaria leafspot and head rot:

In general, it is difficult to manage this disease with fungicides because it appears during wet periods. In our field we had to wait almost a week before we were able to spray because of rainfall.

This is from the Resource Guide for Organic Insect and Disease Management. Copper compounds are labeled, but have not been effective in recent studies (two poor results).

The fungicide options below are from the Commercial Vegetable Production Recommendations guide for Pennsylvania.

Use one of the following at the first sign of disease and continue every 7 to 10 days.

  • Azoxystrobin – 6.0 to 15.5 fl oz 2.08F/A or other labeled formulations
  • Fontelis – 14.0 to 30.0 fl oz 1.67SC/A
  • Cabrio – 12.0 to 16.0 oz 20 EG/A
  • Endura – 6.0 to 9.0 oz 70WG/A
  • Chlorothalonil – 1.5 pt 6F/A or other labeled formulations
  • Ridomil Gold Bravo – 1.5 lb 76.5WP/A (14-day schedule)
  • Switch – 11.0 to 14.0 oz 62.5WG/A

Materials with different modes of action (Fungicide Resistance Action Committee–FRAC code) should be rotated.

Contact Information

Elsa Sánchez
  • Associate Professor of Horticultural Systems Management
Email:
Phone: 814-863-2433
William Lamont
  • Professor of Vegetable Crops
Email:
Phone: 814-865-7118