Main symptoms

Guignardia bidwellii , like Phomopsis viticola , mainly attacks all young organs of the vine in the active growth phase, and in particular the leaves and clusters.

• Leaves

Symptoms first appear on the lower leaves of the vine, as their environment is more humid. Small spots, which can reach 10 mm in diameter at the end of the course, appear on them, generally two weeks after infection; they are slightly chlorotic at first, and circular to more or less polygonal (figure 1). They gradually spread and take on a first gray color then light brown, while their periphery is bordered by a dark brown border (Figures 2 and 3). When conditions are favorable, tiny black spherical structures visible to the naked eye quickly form on the upper surface of the leaves (Figures 4 and 5): these are the imperfect shape pycnidia of the fungus. These are arranged concentrically around the periphery of the spots, or sometimes linearly along the veins. The number of spots present on the leaves can vary from a few lesions to almost complete coverage of the affected leaf blades.

• Twigs, petioles and tendrils

Elongated, depressed spots are also visible on the petioles, raceme peduncles and tendrils (Figure 6); they are chlorotic with a black center. During their extension, the infected tissues turn black-brown and become covered with pycnidia. The young twigs, (Figures 7 and 8) less frequently affected, show comparable canker lesions varying in length from a few millimeters to more than 2 cm. Pycnidia easily form on these lesions (Figure 9).

• Bunches and berries

The berries show the most spectacular damage. They are sensitive from fruit set to the cluster closure stage and are no longer receptive beyond veraison. The first spots appear at half veraison (figures 10 and 11); they are discolored, circular (figure 12) then depressed (figure 13). These spots grow and take on a characteristic livid brown-red color (figure 13) (symptoms not to be confused with brown burping caused by downy mildew). Whole berries spoil very quickly, then they shrivel and mummify in 3 to 4 days (Figures 14 to 16). Their color becomes very particular: black with bluish reflections. Black dots appear on the surface; it is the pycnidia that give the bay a rough appearance (Figures 17 and 18). The cluster may be affected in whole or only in part; the berries may fall to the ground or remain on the plant. If the weather is rainy, they can rot.

• Organoleptic impact

The organoleptic impact on wines from harvests affected by black rot was studied. It is comparable to that caused by gray rot . For example, an attack of about 30% on Merlot results in a reduction of 39% in color intensity, 29% in the level of anthocyanins and 7.5% in tannins. On tasting, the wines are less colorful, on the nose there is a lack of freshness and nuances of overripe fruit. To the taste, the tannins are less well constituted, dry and less melted.