Zucchini
Anthracnose of Cucurbits
Glomerella lagenarium
Fungus
In a Nutshell
- Water-soaked, yellowish circular spots on leaves.
- Circular, black, sunken spots on fruit.
- Girdling and wilting of stems.
Zucchini
Symptoms
Leaf symptoms begin as water soaked lesions that later become yellowish circular spots. The main characteristic of these spots is that they are irregular and turn dark brown or black as they enlarge. Stem lesions are also visible and as they grow, they can girdle the vascular tissues and cause stems and vines to wilt. On the fruits, large, circular, black and sunken spots appear and later become cankers. On watermelon the spots may measure 6-13 mm in diameter and up to 6 mm deep. When moisture is present, the black center of the lesion is covered with a gelatinous mass of salmon-colored spores. Similar lesions are produced on muskmelon and cucumber. Cankers with this pinkish color are the most characteristic symptom of this disease in cucurbits.
Recommendations
Organic Control
Organically-approved copper formulations can be sprayed against this disease in cucurbits and has shown good results in the past. Formulations containing the biological control agent Bacillus subtilis are also available.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatments if available. Apply approved fungicides to the crop at regular intervals, more often if frequent rains occur. Use fungicides containing chlorothalonil, maneb and mancozeb formulations. The combination of chlorothalonil with mancozeb has shown to be very effective when used as a foliar spray .
What caused it?
The symptoms on leaves and fruits are caused by the fungus Glomerella lagenarium, which overwinters on diseased residue from the previous crop or may be carried on cucurbit seeds. In the spring, when the weather becomes more wet, the fungus releases airborne spores that infect vines and foliage close to the soil. The life cycle of the fungus largely depends on ambient humidity, leaf wetness and fairly high temperatures, 24°C being considered optimum. Spores do not germinate below 4.4°C or above 30°C or if they are not supplied with a film of moisture. In addition, the pathogen must have water to free the spores from their sticky covering in the fruiting body. This explains why anthracnose usually becomes established in mid-season after the plant canopy has developed.
Preventive Measures
- Use certified, disease-free seeds.
- Choose resistant varieties, if available in your area (several have been put on the market).
- Avoid the movement of machinery or workers in the fields when the foliage is wet.
- If overhead irrigation is necessary, plan it during the early morning and make sure the foliage is dry before nightfall.
- Rotate cucurbits with unrelated crops in a three-year rotation.
- Practice good sanitation of the field by plowing under fruits and vines at the end of each season.
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