Frogeye leaf spot

small round spots scattered on the upper leaves. Each spot is about the size of a match-head to a small pea, with a grey to light-brown dead centre ringed by a dark reddish-brown border — like a little eye staring back at you. That eye-in-a-ring look is where the name frogeye comes from.

the spots multiply and run together into larger dead brown patches, the leaf turns yellow around them, and badly hit leaves dry up and drop early. In a wet spell, turn a spot to the light and you may see a faint grey, dusty fuzz on the underside of the dead centre — that is the fungus making fresh spores.

late in the season look for grey to dark-brown blotchy patches on the pods, and on the beans inside, grey or brown smudges over the seed coat. Spotted seed carries the disease into next year's crop.

with other leaf spots. Frogeye spots have a clean grey centre with a sharp dark-red ring and sit mostly on the younger top leaves. Bacterial spots look wet, greasy and angular (boxed in by the leaf veins) and often have a yellow halo, not a grey centre. Simple sun-scorch or nutrient burn has no dark ring and no dusty fuzz underneath.

this disease hits during mid- and late season — from flowering through pod-fill — and it loves warm, wet weather (about 25-30 C with heavy dew, frequent rain or overhead watering, and muggy 90%-plus humidity). Start checking at flowering and step up your walks after every warm rainy spell.

every week, walk a big "W" shape so you cover all four corners and the middle — not just the row nearest the path where you always look. Make about 10 stops along the W.

look at the upper and middle leaves of a few plants — the fresh top growth, not just the old bottom leaves. You are hunting for the small grey-centred, dark-ringed frogeye spots, and checking whether the newest top leaves are now getting hit (that means it is climbing and still active).

the danger sign is new spots showing up on the upper-canopy leaves during flowering to pod-fill in warm, wet weather. In plain terms — if you start finding frogeye spots on the top leaves of about 1 in 10 plants while the weather is warm and wet and the crop is flowering to filling pods, act now, before it races up the whole canopy.

choose a soybean variety bred to resist frogeye and suited to your area, and always plant clean, disease-free seed — never keep seed from a spotted, infected crop. Resistant seed is the single most reliable control and it costs you nothing extra at spraying time.

rotate away from soybean for at least one full season onto a non-host crop like maize or a cereal, so last year's fungus in the residue dies off. Plough or dig the old soybean stalks down after harvest so infected trash rots away instead of throwing spores onto the new crop. Do not overhead-water late in the day (leaves that stay wet all night feed the fungus), and don't walk or work through the field while the leaves are wet — you carry spores from plant to plant on your hands and clothes.

if you grew a susceptible variety and spots are climbing onto the top leaves in warm, wet weather during flowering to early pod-fill, a triazole fungicide can hold it back (a triazole is a fungicide that moves a little way inside the leaf to protect new growth). Spray at early flowering to early pod-fill, when the disease is just developing — spraying once the whole canopy is already brown is money wasted.

cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind at your back so the mist blows away from you; keep it off food, off drinking water, and away from children and bees; wash yourself and your clothes afterwards; and respect the pre-harvest interval — the number of days the label says you must wait between the last spray and picking your beans, so no chemical is left on the crop you eat or sell.

don't lean on one chemical group over and over. Rotate to a different fungicide group each time (and avoid relying only on the strobilurin group, which frogeye has already learned to beat in many places) so the fungus doesn't become immune to your spray.

if spots still climb after you have sprayed and the weather stays warm and wet, do not just spray again and again — check you used a resistant variety and a rotating fungicide, and accept that once most of the upper leaves are dead the pods will no longer fill. At that point cut your losses on this crop: harvest what is sound, keep none of the spotted seed for planting, and plan next season around a resistant variety plus rotation so you start clean.