Hello Martha,

Cornflowers are noted for having bacterial leaf spot. It starts out as a small black or brown spots on the leaf, appearing oblong as it grows. The spots are often surrounded by a yellow halo. A cornflower can die from bacterial leaf spot as it turns entirely black.

You may need to prune affected leaves as they appear throughout the season. Wipe the blades of your pruning shears with a cloth dipped in rubbing alcohol to avoid further contamination. Leaf spot thrives in moist conditions avoid wetting the foliage when watering. A well drained soil is essential. If needed add a layer of compost or manure to soil.

The following publications provides further information.

https://www.canr.msu.edu/resources/smart_gardens_begin_with_healthy_soil

http://www.uky.edu/Ag/PAT/cat1/leafdis.htm

Bacterial leaf spot and rust are similar.  You are correct with the same sanitization process.  

Hello Martha,

You are correct a weird pest is on your cornflowers. This pest is called a psyllid... actually, the nymph referred to as a Rudbeckia Triozid called Bactericera antennata. Control measures include handing picking the leaves and destroy them. It appears since the insect is inside the leaf, control measures should be done now thus inhibiting it from pupating to become an adult. In Spring of next year using a systemic insecticide granules to prohibit the pest from pupating is best. Although systemics can also harm bees feeding on flowers. See attached publication.

https://content.ces.ncsu.edu/rudbeckia-triozid