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Symptoms and Signs

Symptoms vary somewhat among hosts, and none is absolutely diagnostic. Premature foliar chlorosis and necrosis and vascular discoloration in stems and roots, however, are characteristic of all hosts. Symptoms of wilting are most evident on warm, sunny days.

Symptoms, Potato

Foliar symptoms first appear as chlorosis and necrosis beginning in the lower leaves. On warm, sunny days, leaves may appear limp and flaccid. Sometimes symptoms occur on only one side of the leaf or the plant (Figure 1). In severely diseased plants, medium-tan discoloration of the vascular tissue is evident (Figure 2), and the plants may be stunted. Tubers of some cultivars may develop a light brown discoloration of the vascular ring, although other factors may cause this symptom. Tuber yield is reduced because of the decreased rate of photosynthesis and premature death of foliage. The optimum temperature range for potato growth is 18-20° C (64-68° F). When the temperature rises above 20° C (68° F), plant stress increases and symptoms of Verticillium wilt are more severe.

Figure 1

Figure 2

Symptoms, Peppermint

Symptoms first appear in the foliage at the top of the plant (Figure 3). Symptomatic upper leaves are sickle-shaped and initially chlorotic or red, soon becoming necrotic (Figure 4). Premature defoliation and death of the plant can result. Vascular discoloration in stems and roots may be observed and diseased plants are often stunted. Rhizomes produced in the fall are weakened and may be killed in the winter.

Figure 3

Figure 4

Symptoms, Maple

In woody perennials such as maple, symptomatic leaves are chlorotic and necrotic, dry, and smaller than normal. Symptoms often occur on only one or several branches (Figure 5). The sapwood is brown in discoloration (Figure 6). Diseased trees may have sparse foliage, branch dieback, buds which fail to open in the spring, and/or fall coloration that develops a few weeks preceding normal autumn coloration. Infected trees may die; however, sometimes trees can be saved if symptomatic branches are pruned.

Figure 5

Figure 6

Diagnosis

Since fungal structures are not visible on most diseased specimens, confirmation of Verticillium wilt requires the use of simple laboratory techniques. Small, thin pieces of infected vascular tissue can be placed onto a culture medium such as streptomycin water agar or Sorensen's NP-10 medium and incubated for 4 days. When the fungus grows out of the vascular tissue it can be examined microscopically (Figure 7). This technique is commonly used in plant disease clinics.

Figure 7

Signs

Small, black, thick walled resting structures, called microsclerotia, are formed in the dying tissue (Figure 8) or live tissue (Figure 9) and are frequently visible with a hand lens. Even though microsclerotia form on senescent tissues in most hosts, they are readily apparent in some hosts on live tissue as well.

Figure 8

Figure 9

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