Didymella bryoniae is primarily a parasite of the aerial parts of cucurbits, especially leaves and fruits. Like Botrytis cinerea , it is an opportunistic fungus which will often benefit from particular conditions (excessive humidity in the shelter, stagnant open water on the leaves and in particular at the edge of the leaf blade, presence of numerous size wounds or numerous senescent tissues. ) to settle on plants and cause serious damage. Symptoms can appear in nurseries, after planting, and more frequently on adult plants.

As this fungus is transmissible by seeds (figure 1), the seedlings can be attacked early from germination. Moist, dark spots can develop on cotyledons (Figure 2), stems, and cause seedling death.

D. bryoniae is able to produce spots on the leaves of several cultivated cucurbits. The spots observed are initially more or less circular, oily to livid. Rather circular in shape, their size fluctuates depending on the plants and environmental conditions. They spread as they evolve, take on a brown tint, necrotize and dry out (figure 3). Note that they are often located at the edge of the limbus at first, and that a yellow halo can sometimes surround them.
During severe attacks in the open field in particular, rapid and significant defoliations were observed, in particular on watermelon, subsequently promoting sunburn on fruits and reducing their sugar content.

Limited lesions, initially moist and dark green, develop on the stems (Figures 4 and 5) (aerial parts and neck) gradually turning brown. They very often start from size wounds, grafting or senescent tissues present on the stem. These lesions spread, more or less rapidly depending on the age of the plants, and sometimes split longitudinally. They can cover large portions of the stems. At the end of their development, they end up encircling them, leading to the dieback of the distal end of the affected branch, the leaves of which wilt and dry up. Finally, note that this fungus is also capable of attacking parts of stems close to the ground or buried. Thus, we can sometimes observe lesions at the bottom of the stem, at the level of the neck, and sometimes on the roots.

The presence of brown exudates , sticky is a fairly consistent feature of stem lesions associated with D. bryoniae . These are clear at first, then later turn brown by oxidizing in air. Let us note that their quantity is sometimes important and that one can also observe some on the peduncles of the fruits in particular. Let us add that this phenomenon of "gummosis" is in fact a fairly frequent symptom in Cucurbitaceae which can accompany many phytosanitary problems, but more particularly those linked to vascular fusarium wilt of melon and attacks by D. bryoniae or Macrophomina phaseolina .

Note also the existence of tiny and numerous brown to black globular masses , which marbling the lesions on stems and petioles (figure 6) in particular and contribute to giving them a dark gray to black tint) . They are very characteristic of D. bryoniae and materialize the fruiting bodies of this fungus: perithecia and pycnidia visible with a magnifying glass. They form on the surface of altered tissues present on the stems, but also on the leaves and fruits.

The symptoms visible on the fruits of cucurbits are strongly influenced by the behavior of opportunistic parasites of D. bryoniae . Indeed, the black rot that it causes is often initiated from more or less senescent floral parts (or in areas of the fruits where these are found - stylar area), or micro injuries (peduncular area). . Note that the young fruits are rather more sensitive. Thus, initially small, damp and dark lesions develop on the fruits (Figure 7), and gummy exudates are formed. Large lesions can initiate on contact with the ground (Figure 8). Irregularly shaped, they gradually spread and darken. In some cases, the development of D. bryoniae can be insidious and mainly takes place inside the fruits (figure 9). Quite quickly, the tissue turns black, remains momentarily firm, and the lesion spreads. Cucumber is particularly affected by this type of symptoms, often showing a pinching at its tip. Watermelon, squash that are stored for long periods of time, are also susceptible to attack by D. bryoniae . The squash reveal a black rot ( black rot ) during storage, it spreads inside the fruits.

Note that the lesions can be invaded secondarily by Pectobacterium carotovorum subsp. carotovorum , bacteria that amplifies the rot in place.

Last change : 05/03/21

• Author :
• D Blancard (INRAe)