In August in early onion crops, if weather conditions are conducive, the first signs of downy mildew are not uncommon and, in recent seasons, Stemphylium leaf blight has also been seen. This is a key time to make sure crops are checked regularly for signs of diseases before they get a strong hold.

Downy mildew, Peronospora destructor, is the most significant foliar disease in alliums, causing pale yellow and/or light green oval-shaped lesions on leaves. In humid conditions, these lesions show masses of grey fungal spores. Lesions quickly turn brown and leaves will collapse. Periods of dew with temperatures between 7 to 16°C promote germination of spores, increasing infection rates.

Until recently, Stemphylium leaf blight, Stemphylium vesicarium, was not considered a major disease in onions. However, it looks like it is now here to stay in New Zealand. Stemphylium often requires an entry point into the plant such as disease, herbicide burn or stress. Therefore, taking steps such as good management of other plant diseases, managing herbicide use to reduce burn and promoting good healthy growth of plants with proper nutrition help to reduce plant stress and lessen the risk of Stemphylium infection.

Focus your fungicide programme on the preventative control of these diseases. Use products such as Zorvec Enicade or Revus prior to the disease being found in the crop, then switch to an alternative, robust, systemic fungicide as part of good resistance management. A strong option for downy mildew is Reason, and Zampro offers a useful option if you’re looking to reduce your Mancozeb use. Apex now has a registration for Stemphylium leaf blight control in onions, offering another option.

For more information on early disease control in vegetable crops, please contact your Fruitfed Supplies Technical Horticultural Representative.