Graminicolous Downy Mildews

Downy mildews pose a significant threat to our agricultural grains (e.g. corn, oats, wheat), but if an outbreak occurs, we don’t have a good molecular and morphological database available to identify the species responsible. The goal of this project is to build that database.
But, we need help collecting samples! Please be on the look out for agricultural grains and native/weedy grasses infected with downy mildews and send them to us.
Peronosclerospora philippinesis
Peronosclerospora philippinesis

Chlorosis of corn leaves caused by Peronosclerospora philippinesis. Image courtesy of Bob Kemerait, University of Georgia, Bugwood.org, used with permission.

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Peronosclerospora philippinensis
Peronosclerospora philippinensis

Chlorosis of corn leaves caused by Peronosclerospora philippinesis. Image courtesy of Bob Kemerait, University of Georgia, Bugwood.org, used with permission.

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Peronosclerospora sorghi
Peronosclerospora sorghi

Characteristic downy appearance of Peronoscleropora sorghi on sorghum leaf. Image courtesy of Parthasarathy Seethapathy, Tamil Nadu Agricultural University, Bugwood.org, used with permission.

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Peronosclerospora philippinesis
Peronosclerospora philippinesis

Chlorosis of corn leaves caused by Peronosclerospora philippinesis. Image courtesy of Bob Kemerait, University of Georgia, Bugwood.org, used with permission.

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Sclerospora graminicola

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Sclerospora graminicola infects a variety of Setaria spp., including the domesticated foxtail millet (S. italica). It can also infect pearl millet (Pennisetum glaucum), and it causes great losses of this crop in India. However, strains infecting pearl millet are restricted to that host, and strains on Setaria spp. are restricted to that genus, which suggests S. graminicola may be a species complex. 

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Setaria viridis, the main host of S. graminicola in the USA, is an annual weed that shows strong local adaptation. For example, in the figure above, modified from Schröder et al. (2017), North American S. viridis forms two distinct populations corresponding to a shift from a warm temperate climate with hot, humid summers to a snow based climate with warm, humid summers. This offers an opportunity to test hypotheses of sequential speciation. We predict that North American S. graminicola populations will follow S. viridis populations and form two distinct clusters. To test this, we need help collecting samples of infected Setaria virdis. Please keep a look out!

Crazy top of pearl millet
Crazy top of pearl millet

Crazy top of pearl millet caused by S. graminicola. Image courtesy of Parthasarathy Seethapathy, Tamil Nadu Agricultural University, Bugwood.org, used with permission.

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Crazy top
Crazy top

Crazy top of pearl millet caused by S. graminicola. Image courtesy of Parthasarathy Seethapathy, Tamil Nadu Agricultural University, Bugwood.org, used with permission.

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Chlorosis of leaves
Chlorosis of leaves

Chlorosis of pearl millet leaf caused by S. graminicola. Image courtesy of Parthasarathy Seethapathy, Tamil Nadu Agricultural University, Bugwood.org, used with permission.

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Crazy top of pearl millet
Crazy top of pearl millet

Crazy top of pearl millet caused by S. graminicola. Image courtesy of Parthasarathy Seethapathy, Tamil Nadu Agricultural University, Bugwood.org, used with permission.

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Sclerophthora macrospora 

Crazy top of corn & wheat

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Image courtesy of Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org, used with permission.

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Image courtesy of Brian Olson, Oklahoma State University, Bugwood.org, used with permission.

Sclerophthora macrospora causes crazy top of corn and wheat, which greatly reduces yields. It also causes diseases in rice and turf grasses.

Crazy top of wheat
Crazy top of wheat

Image courtesy of Brian Olson, Oklahoma State University, Bugwood.org, used with permission.

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Leaf chlorosis
Leaf chlorosis

Leaf chlorosis of a rice plant infected with Sclerophthora macrospora. Image courtesy of William M. Brown Jr., Bugwood.org, used with permission.

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Oospores in corn leaf
Oospores in corn leaf

Sclerophthora macrospora oospores in a corn leaf as seen under a microscope. Image courtesy of Brian Olson, Oklahoma State University, Bugwood.org, used with permission.

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Crazy top of wheat
Crazy top of wheat

Image courtesy of Brian Olson, Oklahoma State University, Bugwood.org, used with permission.

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Native & Weedy Grasses

Downy mildews, particularly Sclerospora graminicola and Sclerophthora macrospora, also infect a variety of grasses that are native and introduced weeds to North America. The symptoms will be similar to those listed above for the agricultural grasses, but they will be less pronounced.

 

Here is a list of some grass species known to be a host and a link to their USDA Plant Profile page:

This list is not at all exhaustive! A more complete list can be found here. If you see a grass that may be infected with downy mildew and you are able to collect it, feel free to send it to us.