Some of the most devastating diseases of sorghum and maize are caused by downy mildews.

There are at least five exotic species of downy mildews that infect sorghum and maize overseas which have not been found in Australia. Two species – sorghum downy mildew (Peronosclerospora sorghi) and Philippine downy mildew of maize (Peronosclerospora philippinensis) – are classified as high-priority exotic plant pests of the Australian grains industry.

Other exotic downy mildew pathogens, including Peronosclerospora maydis, also pose a threat to the Australian summer grains industry.

Recent surveys conducted by Queensland researchers Professor Malcolm Ryley, Professor Roger Shivas and Dr Yu-Pei Tan have revealed that Australian native grasses are also host to a diversity of downy mildews.

Hidden diversity

Professor Shivas, a plant pathologist at the University of Southern Queensland’s Centre for Crop Health, says studies of downy mildews on Australian grasses have identified 17 species new to science over the past 15 years.

“This brings the total number of downy mildews on grasses in Australia to 20, most of which were found on Australian native grasses,” Professor Shivas says.

“These results are significant for both scientific and biosecurity reasons. A knowledge of the downy mildews on native and other grasses provides us with a baseline of what species we have in Australia."

"This information will assist us to rapidly identify the downy mildews responsible for future outbreaks on maize, sorghum and other crops should they occur."

“For example, intermittent outbreaks of downy mildew on maize have occurred over the past 40 years at locations in tropical northern Australia. These were often attributed to P. maydis.

A knowledge of the downy mildews on native and other grasses provides us with a baseline of what species we have in Australia.

But, Professor Shivas says the results of the studies indicate that the outbreaks on maize in northern Australia were likely caused by infection from spores of Peronosclerospora australiensis - which occurs widely across Australia's top end.

An eye on symptoms

In order to keep Australia’s cropping regions free of these potentially devastating diseases, it is important for researchers, growers and agronomists to be aware of the symptoms and report any suspected detections.

Sorghum and maize plants that develop from infected seeds are stunted and malformed.

The first symptoms of downy mildew on sorghum and maize are long, wide, parallel-sided yellow streaks on leaf blades.

These streaks are sometimes confined to the lower part of leaf blades near the stem. It must be noted that yellow streaks on leaf blades of maize, sorghum and other grass hosts may also be caused by nutritional disorders or viruses.

A clear sign of downy mildew is the development of a white superficial growth on the lower leaf surface. This down often develops in the moist early hours of the morning and dries rapidly as the air temperature rises.

Some species of downy mildew also cause leaf shredding as the plants mature due to the formation of spores between the leaf veins.

How it spreads

Downy mildew can be spread via soil, airborne spores or infected seed. Consequently, farm biosecurity practices such as cleaning vehicles and equipment play an important role in limiting spread.

Mature spores detach from the leaf and are spread short distances by wind. Those spores that successfully land on a leaf need specific levels of moisture and temperature in order to germinate and infect the plant.

Some species of downy mildew also produce thick-walled spores that form between the parallel veins of the leaf and can survive in the soil for at least five years. These thick-walled spores cause the leaf to shred and form tassels.

Reporting

Timely reporting of a detection of downy mildew may help with its eradication or control.

If you suspect you have found an exotic downy mildew, report it to the
Emergency Plant Pest Hotline, 1800 084 881.