FUSARIUM WILT (vine) and DRY ROT (tuber)

Several species of Fusarium cause Fusarium wilt on vines and dry rot on tubers.

I. On vines, the disease is called Fusarium wilt.

Symptoms begin first with a general wilting of the plant. A rosette appearance of the vine top and leaf discoloration can occur in severe cases. Upon cutting a cross-section of the stem, the whole vascular ring (unlike the early symptom of Verticillium wilt described below) would be discolored, brown to black. At the base of the vine, the stem is brown to black but not in blotches as with stem canker (see last month's column). Stem decay at the base is from the vascular tissue out and in, unlike stem canker which is from the outside (epidermis) through the vascular tissue and goes further in. At later stages these two disease cannot be told apart based on stem appearance.

II. On tubers, the disease is called dry rot or Fusarium decay.

a. There are surface discoloration which grow deeper into the tuber. The stem-end discolors. Starting at the stem-end of the tuber, peel thin (eighth of an inch at a time) layers. A dark ring (infected vascular ring) may appear; this is the disease in the vascular tissue. But, note this alone may not indicate Fusarium decay since this symptom can appear due to a too rapid vine kill, Verticillium wilt, bacterial ring rot, and other causes.

b. There is also a specific disease on tubers called dry rot which is caused by Fusarium solani. Organism enters through wounds on the tuber surface. It spreads from the point of entry leaving a cavern of dried dead tissue. Under high humidity, bacterial soft rot will also attack and the cavern becomes a liquid/dry mixture. There is an odor from the soft rot but not a stench like in Pythium leak which smells like "dead mackerel."

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