Broccoli is affected by various diseases throughout the growing cycle and also later in the packing and distribution phases. An important disease to be aware of is Alternaria head rot, also known as pin rot. The fungal disease is caused by different Alternaria species, Alternaria brassicae, A. brassicicola and A. japonica.
Fungal or bacterial?
Initial symptoms appear very similar to those of another disease, bacterial head rot. Both appear as a water-soaked discoloration of single or small groups of florets which later turn brown or black. Heads infected with the bacterial disease tend to show a softer rot with a pronounced smell.
Secondary fungal and bacterial pathogens can further confuse the picture and analysis of infected tissue by an experienced pathologist will give the most accurate diagnosis. Alternaria infection on leaves appears as round, papery, target-spot lesions containing black, sooty spore masses during moist conditions. The centres of older lesions fall out and create a shot hole appearance. These symptoms are distinct from those of downy mildew (Hyaloperonospora parasitica) where the upper leaf surfaces show yellow to brown spots and the lower surfaces a corresponding grey to white mold under moist conditions. Older downy mildew lesions remain intact, often containing brown spots. Again, accurate identification is important so that the correct control measures can be applied.
Early detection of Alternaria leaf spot by scouting for the small brown to black dots with chlorotic haloes will lead to more effective chemical control. Symptoms will first be found on the older, lower leaves progressing up the plant towards the head.
Climatic conditions that favour disease development include high humidity and long periods of leaf wetness with temperatures of 20°C to 30°C. However, infection can occur at temperatures as low as 13°C. Alternaria produces large numbers of airborne spores that are generally spread by wind but also by water splash, insects, equipment and movement through a crop. Alternaria can survive on previous crop debris and some weed species to provide inoculum for subsequent plantings.
