CDM Diagnostic Key – Question 2 – No

If dark gray-brown to purplish sporulation is not present:

1.   CDM pathogen spores are not always present. If spores are not present, place symptomatic leaves in a plastic bag with a damp paper towel for 12 – 24 hours and spores may form.

If sporulation develops, answer yes for Question 2. If sporulation does NOT develop, proceed below.

2.   If sporulation does not develop, the plant could be infected with a bacterial disease like angular leaf spot

Fig. 10. Angular leaf spot foliar symptoms (Beth K. Gugino, Penn State)

Angular leaf spot is a bacterial disease that produces angular lesions that are initially water-soaked later becoming tan-brown and surrounded by a yellow halo. Angular leaf spot can be differentiated from CDM because it is caused by a bacterium and will not produce spores on the underside of the leaf.

3.   If the sporulation is white, it may be powdery mildew

Fig 13. Powdery mildew on underside of pumpkin leaf (Beth K. Gugino, Penn State)
Fig 14. Downy and powdery mildew sporulation on underside cucumber leaf (Beth K. Gugino, Penn State)

Powdery mildew fungi produce masses of white spores on the upper and lower leaf surfaces whereas CDM produces dark gray-brown to purplish spores on the lower leaf surface. Powdery mildew lesions are not limited by leaf veins like with CDM.