Pale Areas, Yellow Spots on Leaves Caused by Fungal Disease.
Downey Mildew causes mottling and pale areas on upper leaf surfaces and a fuzzy mold growth on their undersides. Eventually they wither and die. The oldest leaves are usually infected first. Downy mildew is most likely to occur on cool wet nights and warm humid days. It spreads by means of tiny spores carried to plants and soil by wind and rain or transmitted by your tools. It will not kill a mature plant; it just mars its appearance.
Rust appears on upper leaf surfaces first as yellow or white spots that turn brown or black. Puffy blisters then appear on their undersides. The disease may spread to stems and flowers, causing distorted growth. It sometimes spreads to the cultivated sunflowers from weeds such as wild mustard, shepherd’s-purse, pigweed, and lambsquarters.
If you spot fungal diseases early you can protect healthy foliage from infection by spraying it with a general garden fungicide as directed on the product label. Remove and destroy seriously infected plants. Clean up plant debris from the garden in the fall. Disinfect tools by dipping them in a solution of 1 part household bleach to 4 parts water. Keep your hands clean, and do not handle plants when they are wet. For more information see the file on Controlling Fungal Disease
Plant Stems Rot at Soil Line from Damping-Off Disease
Caused by various fungi, this disease attacks stems and flower heads of young plants. It kills them by rotting the stems at the soil line so they topple over and die. The fungus lives in the soil and loves high humidity and areas that have poor air circulation.
To minimize infection risk, make sure your soil is well-drained and warm before planting the seeds. Also try spreading pasteurized soil or a sterile vermiculite medium in furrows where you plant your seeds. Avoid watering past noontime so that the plants and soil surface will be dry by evening. Check to see that your planting site is open to adequate air circulation.
Flower and Seeds Damaged by Sunflower Moth
This small gray moth lays its eggs in the developing flowers. The emerging larvae are greenish-yellow larvae with 5 brown stripes down the back. They feed in the flower and destroy seeds, creating a mass of webbing and debris.
Hand-pick the worms from the plants and mash them between your fingers or by dropping them into a plastic bag for the trash. If lots of plants are infested, spray or dust the flowers with a product containing Bacillus thuringiensis as soon as you see the larvae. They will eat the bacterium and soon die. Because Bt is rapidly inactivated by sunlight and rain, respray as directed on the product label.
Seed Heads Damaged, Stalks Broken by Birds or Squirrels
Birds and squirrels can be a problem when seeds ripen and harvest time approaches. If you do not plan to use the seeds, it is fun to watch the birds and squirrels enjoy them. You may want to cut them off and lay them out in the sun to dry to provide easier access to wildlife.
To deter birds and squirrels, barrier devices are most effective. As seedheads mature and flowers droop, cover each one with white polyspun garden fleece. It will let light and air in and keep critters out. Also try cutting away the few leaves that are closest to the heads to make it harder for birds to perch and feed. For more information see the files for Controlling Pest Birds and Dealing With Squirrels