Citrus powdery mildew

Citrus powdery mildew is caused by the fungi Oidium citri and O. tingitaninum.

Powdery mildew on other crops, though similar looking, is caused by different species of powdery mildew fungi.

Leaves

• White 'powdery' spores develop mostly on the upper leaf surface.
• Young leaves turn a pale whitish-grey-green.
• Leaves and shoots can distort.
• Young shoots can wither and die.
• Severe infection causes defoliation.

Flowers and fruit

• White 'powdery' spores develop on flowers and on young fruit.
• Infected fruit fall prematurely.

Trees

• Plants can develop twig and branch dieback.

Plant stage and plant parts affected

• Infection usually appears first on new flushing leaves.
• All aerial parts of the plant may be affected.

The disease is common in parts of Asia, including South-East Asia (Java, Sumatra). There have also been reports from Uganda, Israel, Central and South America, the United States (California), the Republic of Honduras and Antigua.

Citrus powdery mildew is not known to occur in Australia.

All citrus cultivars can be affected. Some cultivars appear more susceptible than others. In India, the citrus varieties that are most susceptible are mandarins, sweet oranges and tangerines.

Citrus is a significant crop in Australia, there is over 28,000 hectares of citrus planted and around 1,900 growers. For the year ending June 2017, citrus production was valued at $724.4 million, with Queensland the largest producer of mandarins.

Citrus powdery mildew can cause premature leaf and fruit drop, and twig and branch dieback. Severe infection can significantly reduce tree productivity, fruit quality and yield. Farmers could experience production losses and increased costs associated with managing the disease. Market access could be disrupted.

Backyard citrus growers would also be affected.

This disease produces tiny, powdery spores that can survive on fallen leaves.

The spores can be transported:

• by wind
• on people (clothing, hands)
• on equipment (e.g. pruning tools, mechanical harvesters or hedgers)
• on vehicles.

Movement of infected citrus plant material poses the greatest risk of introducing citrus powdery mildew to Australia. The Australian Government closely regulates approved host plant imports and monitors for illegal plant movement.

Check crops often. Look for plants that are unthrifty and that may also have dieback and fruit drop. Examine leaves, shoots, branches, flowers and fruit for white powdery patches of spores and distortion.

If detected, report immediately to Biosecurity Queensland.

The key method of preventing citrus powdery mildew entering your farm is to use of certified disease-free planting material and have robust on-farm biosecurity systems in place.

Find out how you can protect your farm from plant pests and diseases:

• Biosecurity manual for citrus producers (PDF, 4MB)
• Protect your farm from emergency plant pests
• Understand good farm biosecurity
• Visit the farm biosecurity website

Citrus powdery mildew is an exotic pest of citrus.

Report suspected citrus powdery mildew to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

If you think you have found citrus powdery mildew, you must take all reasonable and practical steps under your control to minimise any associated risks. This is called a general biosecurity obligation (GBO).