The fungus Rhizoctonia can be a nuisance to many vegetable growers. Readers may recognize this unusual name as a cause of many diseases such as damping off and root rot in many different crops. This article is about Rhizoctonia as the cause of wirestem in broccoli.
Rhizoctonia may affect several brassica crops such as broccoli, cauliflower, Chinese cabbage and radish. Recently, a disease known as wirestem was observed on broccoli. The first symptoms that growers are likely to observe is a wilt of very young broccoli plants. When the plants are examined more closely, it can be observed that the stem has rotted away, as if to a wire (Figure 1). This can cause the plant to wilt and collapse.
Figure 1. Wirestem of broccoli. The stem has been washed and the discolored tissue has mostly been removed.
The Rhizoctonia fungus may survive in the soil for long periods without a host. And, as mentioned above, the fungus has many hosts. Rhizoctonia diseases are favored by warm soil temperature (above 68°F) and wet conditions.
When direct seeding, fungicide treated seed is an option that may lessen wirestem severity. When transplanting, avoid transplanting too deep. Avoid brassica crops in crop rotations. Deep tillage practices which reduce crop residue may help to lessen disease. Products with the active ingredient pentachloronitrobenzene (PCNB) (e.g., Blocker 4F®) applied at planting are labeled for wirestem of broccoli. It is difficult to predict, however, when or if such a treatment is warranted.