Didymella stem rot, also known as gummy stem blight, is a widespread and destructive fungal disease that primarily impacts plants within the cucurbit family, which includes cucumbers, melons, squash, and pumpkins. This disease can affect all parts of the plant, from seedlings to mature fruits, leading to significant crop losses if not managed effectively. Its presence is noted globally, indicating its adaptability and pervasive nature in agricultural settings.
Recognizing the Symptoms
On stems, symptoms often begin as pale brown or gray spots that can elongate into streaks, particularly at the joints. These lesions may exude a characteristic amber-colored gummy substance, which gives the disease its common name, gummy stem blight. Severe stem infection can lead to girdling, causing the vine to wilt and eventually die.
On leaves, symptoms appear as circular, dark tan to black spots, often encircled by a yellow halo. These spots may dry out, crack, and fall away, creating a “shot-hole” appearance. Infection at the leaf margins often starts as a wilt and progresses inward, forming V-shaped lesions of dead tissue. Tiny black fruiting bodies, known as pycnidia or pseudothecia, may become visible within the infected leaf and stem tissues.
Fruit symptoms also vary by crop, initially appearing as irregularly circular spots that change from a fading of fruit color to gray or brown. These darkened spots can have a hardened gummy droplet at their center and may turn black, penetrating the rind to form lesions. Such lesions provide entry points for other organisms, leading to a watery rot where the entire fruit can become a liquid mass enclosed by its skin.
Conditions Favoring Development
Didymella stem rot is caused by the fungus Didymella bryoniae. This pathogen survives in infected plant debris from previous cucurbit crops and can also be seed-borne. Wounds on plants, whether from pruning, insect activity, or field work, serve as entry points for the fungus.
Optimal temperatures for disease development range from 20–28°C (68–82°F), with fungal growth possible between 7°C and 33°C. High humidity, especially above 85% relative humidity, combined with prolonged periods of leaf wetness, greatly favors infection. Spores germinate when free water is present on the plant surface.
The pathogen spreads through various means, including airborne ascospores, which can travel between fields, and conidia, which are dispersed over short distances by water splash. Contaminated tools and the hands of workers can also facilitate the movement of spores.
Preventative Measures and Management
Effective management of Didymella stem rot involves a combination of cultural practices and, when necessary, chemical controls. Sanitation is a primary measure, requiring the removal and destruction of all infected plant material, including crop debris and volunteer cucurbits, to reduce inoculum sources. Crop rotation is also beneficial, with a minimum of two to three years of non-cucurbit crops recommended to break the disease cycle.
Improving air circulation around plants through proper spacing helps to reduce leaf wetness, thereby making conditions less favorable for fungal growth. Managing irrigation practices, such as using drip irrigation instead of overhead watering, minimizes the duration of leaf wetness. When possible, selecting and planting disease-resistant varieties provides protection.
While cultural practices are important, chemical controls, such as fungicides, may be necessary as part of an an integrated management approach, particularly if environmental conditions are conducive to rapid disease spread. Fungicides like mancozeb, chlorothalonil, and captan have shown effectiveness in controlling the disease. To prevent the development of fungicide resistance, alternate various chemical classes during a spray program, applying them preventatively during early plant growth stages. Some biological control agents, like certain Bacillus strains, are also being explored to inhibit fungal growth.