Alternaria brown spot

What you see

Brown to black spots with a yellow halo on young leaves and shoots, often with dark necrotic veins radiating out from the spot. On fruit, sunken dark lesions with a corky centre. Heavy young-fruit drop.

Conditions

Warm weather with rain or dew on the new flush. It attacks only young, expanding tissue, so it follows every growth flush.

Treatment

Copper applied to protect each new flush is the practical home control. Remove fallen infected fruit and leaves. Prune out badly affected twigs.

Prevention

Avoid excessive nitrogen, which pushes soft susceptible flushes. Open the canopy so it dries quickly. Grow less susceptible types - it is chiefly a mandarin and tangelo problem.

Months: March, April, May, September, October

Plants

Tangerine, Orange, Lemon

Remedies

Bordeaux mixture (copper), Horticultural oil (white oil)

The thing people get wrong

This one is largely self-inflicted through feeding. The fungus can only infect tissue that is still soft and expanding, so a tree pushed hard with nitrogen produces flush after flush of exactly the tissue it wants. Easing off the feed and accepting slower growth reduces brown spot more reliably than spraying does.