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Symptoms and Signs

Monosporascus cannonballus infects the roots of plants, primarily the secondary and tertiary roots. Infection can occur very early in the growing season, but above ground symptoms are not usually apparent until much later in the season. The primary symptoms are necrosis of the small roots and lesions on the larger roots, typically around the root junctions (Figure1), however the most dramatic symptom is the collapse of the vine late in the season (Figure 2).


Figure 1.		Figure 2.

Root colonization and necrosis continues throughout the life of the plant causing increasingly more damage to the root system. In severe cases, larger roots, including the taproot, become infected. The above ground symptoms displayed as a result of root damage are early season stunting of the plant and a gradual death of the leaves, beginning with the crown leaves. Typically, late in the season, the entire remaining vine collapses (Figure 2). Disease severity is exacerbated by plant stresses, including plant maturity, fruit load, and biotic and abiotic factors, including other soil microorganisms, soil temperature, moisture, and pH. Diseased plants have few secondary and tertiary roots (Figure 1) and may or may not display symptoms of a wet root rot where the cortex of the roots easily sloughs off. There is no indication that wet rot is due to M. cannonballus per se, but rather is most probably the result of secondary fungi or bacteria present in the soil taking advantage of a compromised root and moist soil conditions.

The primary diagnostic sign of the pathogen is the formation of black, spherical, erumpent perithecia on the roots (Figure 3), easily visible with the unaided eye or hand lens. Perithecia are the sexual reproductive structures (fruiting bodies) of the fungus and develop along the entire root length and, at times, can be quite numerous. They are most commonly observed on secondary lateral roots late in the season; however, they typically are more abundant after the plant (root) has been dead for a few days. This may result in the misdiagnosis of the disease or a very late diagnosis, often after the plant has died. Perithecia may be difficult to observe in the field because many of the necrotic roots bearing them are lost prior to or during extraction from the soil. When mature, the perithecia rupture, spilling the ascospores into the soil (Figures 4 & 5).


Figure 3.		Figure 4.		Figure 5.

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