Main symptoms

As we have previously reported, many cucurbits are affected by anthracnose. Colletotrichum orbiculare can attack both nursery seedlings and adult plants, causing symptoms on the aerial organs of plants: leaves, stems, and especially fruits. Note that these symptoms are rather identical between species of cucurbits. It is for this reason that we will not detail them for each of them. Finally, let us add that they appear rather, initially, on a few plants located in foci.

On leaves , this fungus first causes small oily spots (figure 1), quickly turning brown to reddish brown, then necrotic. These sometimes extend onto the veins or start from them (Figure 2). Circular to somewhat angular in shape, their size can exceed 1 cm (Figure 3) and they can be surrounded by a translucent to yellowish halo. Tissues in the center of the lesions eventually dry out and may detach from the limbus.
Note that leaf attacks occur more particularly in open field crops. They can be at the origin of sometimes rapid and significant defoliation, and thus increase the damage on fruits linked to the effects of the sun.
Confusion of diagnosis on leaves is possible with several other aerial diseases of cucurbits: cladosporiosis especially, but also bacteriosis in France, and alternaria and septoria in other countries of the world.

On the stems and petioles , this fungus forms more or less elongated and slightly depressed oily lesions from which a brown exudate sometimes bead (figure 4). If the climatic conditions are favorable, these lesions spread, merge and are also covered with small pink to orange masses (figure 5). Note that they can develop anywhere on the stem and petioles, and weaken them. This is particularly the case when they have given rise to bursts.

It is from the symptoms on fruits that we can easily differentiate anthracnose from the other diseases mentioned above. Small greasy, canker and / or corky spots are first observed. Subsequently, they become round to oval, depressed, healing poorly on the periphery and taking a brown to black tint (Figures 6 and 7). It should be noted that these lesions are often covered by black punctures (stromas) (figure 8) and salmon-pink gelatinous masses (acervuli producing numerous spores) at the origin of the name nuile rouge (figure 7). These fruiting bodies are sometimes arranged in concentric circles and a white mycelial felting is visible on the lesions.

Finally, let us add that if the fruits are affected early, they can be deformed once well developed. In addition, symptoms on the latter can appear both during cultivation and during their storage and marketing.

In order not to confuse cladosporiosis and anthracnose on melon, we advise you to observe the symptoms on fruits (figure 9).