Ring rot – Corynebacterium sepedonicum, bacteria
This is one of the most–feared potato diseases because the causal bacterium is easily disseminated and is responsible for serious market losses. Seed potato certification programs have a zero tolerance for this disease. When infected tubers are cut crosswise, the characteristic creamy yellow to brown breakdown of the vascular ring can be observed. The odorless decay is usually initiated near the stem end. Squeezing the tuber causes a cream–colored, cheesy exudate to ooze from the vascular ring, leaving a distinct separation of the vascular ring and surrounding tissue. In advanced disease stages, further tissue breakdown within and adjacent to the vascular ring by secondary organisms can cause external surface skin cracks, frequently accompanied by a reddish brown discoloration. In general, there is no spread from diseased to healthy tubers while in storage. Tuber symptoms are usually apparent at harvest, but some infected tubers remain symptomless for many weeks in cold storage. Infected seed and dried bacterial slime on crates, bags, cutting knives, storage bins, and machinery may all serve as sources of inoculum for ring rot. Unfortunately, automatic seed cutters and the picker–planter are ideal for spreading the disease. Certified seed and sanitation are of utmost importance for control.