Corn; Sugarcane

No known vector.

Downy (grayish) fungal structures are observed primarily on the underside of the leaves.

Chlorotic stripes/streaks are usually the first symptom observed. As the plant ages, leaves may narrow, become abnormally erect, and appear somewhat dried-out. As the corn plant matures, tassels become malformed and produce less pollen, ear formation is interrupted, and sterility of seeds can result.

There is a synergistic relationship between downy mildew fungi and maize streak virus (MSV) on corn. Infection by MSV can mask symptoms of downy mildew infection.

Preliminary Screening:
Morphological: Species are differentiated only by variations in the size and shape of their conidia (spores) and conidiophores (spore-bearing structure), and sometimes the presence or absence of oospores and differences in host ranges. See Table 2 in Smith and Renfro (1999) for characteristics of downy mildew on corn.

The CAPS-approved diagnostic method requires molecular analysis for species confirmation.

Screening:
Molecular: A Loop-mediated isothermal AMPlification (LAMP) assay is approved for rapid screening of Peronosclerospora philippinensis and P. sorghi. This method distinguishes endemic species, like P. sorghi, from the exotic P. philippinensis. A multiplex conventional PCR protocol is also available for screening.

To request a copy of these protocols, email the S&T Beltsville laboratory at APHIS-PPQCPHSTBeltsvilleSampleDiagnostics@aphis.usda.gov and use the subject line "Diagnostic protocol request".

Confirmation: Suspect positives should be sent to the S&T Beltsville laboratory for confirmatory analysis following the guidelines from the Federal Select Agent Program.

Peronosclerospora spp. and other downy mildew genera (including Sclerospora and Scleropthora).P. philippinensis is indistinguishable from P. sacchari by isozyme analysis and many experts have synomized the two species.

Peronosclerospora philippinensis is on the Select Agent List.