Gadenin日本語Get the appRust, the one you will actually meet
Crown rust is the standard oat disease and in a damp temperate garden you should expect it most years. It shows as small bright orange to yellow pustules scattered on the upper leaf surface, raised enough to feel gritty under a fingertip and powdery enough to leave orange dust on your hand. It begins on the lower leaves and works upward, and the damage it does depends entirely on how high up the plant it gets and how early. Rust confined to the bottom leaves after the grain has filled costs you almost nothing. Rust reaching the flag leaf, the top leaf that feeds the panicle, before the grain has filled costs you a large share of the crop.
Stem rust is the less common companion, appearing as darker reddish brown elongated pustules on the stems and leaf sheaths rather than the leaf blades, and it arrives later in the season. It weakens the straw and contributes to lodging as much as it reduces grain. Both rusts run on warm damp weather with long leaf wetness periods, which is precisely the weather that suits oats in every other respect.
There is no useful cure once rust is established in a garden bed. What works is prevention through timing and airflow: sow as early as the ground allows so the crop fills its grain before the warm humid weather that rust prefers, keep the stand out of a sheltered airless corner, avoid heavy nitrogen which produces soft susceptible leaf, and do not water overhead in the evening. Clear and compost or burn the residue at the end of the season rather than leaving it to carry spores over. Rust also cycles through buckthorn as an alternate host, so a buckthorn hedge next to your grain bed is worth knowing about.
| Problem | What you see | When | What to do |
|---|---|---|---|
| Crown rust | Bright orange gritty pustules on upper leaf surfaces | Warm damp weather from late spring | No cure. Sow earlier next year, improve airflow, clear residue |
| Stem rust | Dark reddish brown streaks on stems and sheaths | Later in the season | Cut the crop as soon as it reaches hard dough, expect weaker straw |
| Loose smut | Panicle emerges as a black powdery mass instead of grain | At heading | Pull and bag affected plants before the dust blows, do not save that seed |
| Bird cherry-oat aphid | Small dark green aphids on leaves and in the whorl | Autumn and spring on young plants | Tolerate a few, hose off or squash colonies on young plants, the virus they carry is the real cost |
| Barley yellow dwarf virus | Yellow or reddish stunted plants in patches, poor tillering | Spring, weeks after aphid feeding | Nothing curative. Sow later in autumn to miss the aphid flight |
| Crown and root rot | Plants yellow and die in patches, browned rotted stem base | Wet cold soil, any time | Improve drainage, do not sow oats in that bed for two or three years |
| Frit fly and stem borers | Central leaf of a young shoot yellows and pulls out easily | Early growth | Tolerate. A healthy tillering stand compensates by throwing new shoots |
| Birds | Panicles stripped, grain gone, stalks bare | Heading to harvest | Net on a frame held clear of the panicles. Nothing else works for long |
Aphids and the virus they carry
The aphids on an oat plant are rarely worth worrying about for the sap they take. What matters is that the bird cherry oat aphid transmits barley yellow dwarf virus, and that virus is one of the few things that can take out a garden oat stand outright. The aphids arrive on young plants, usually in autumn on an autumn sown stand or in early spring on a spring sown one, feed briefly and move on. The symptoms appear weeks later and by then the aphids are gone and there is nothing to treat.
Infected plants are stunted, tiller poorly, and go a hard yellow or in some cases a distinct reddish purple starting from the leaf tips and margins. It appears in patches rather than uniformly across the bed, which is the giveaway that distinguishes it from a nutrient problem or from waterlogging. Plants that are infected young stay dwarfed and set almost no grain; plants infected after jointing carry on more or less normally.
The only real control available to a gardener is timing. On an autumn sowing, going in at the later end of the window means the young plants are exposed to fewer weeks of aphid flight before cold shuts the aphids down. In spring, sowing early gets the plants past the vulnerable seedling stage before the main flight arrives. Beyond that, leave the aphids alone and let the ladybirds, lacewings and parasitic wasps that come to an unsprayed garden do the work, because they will.
A patch of yellow stunted plants is worth marking on a sketch of the bed. Virus patches, crown rot patches and simply wet corners all look similar at a glance, but they behave differently across years: a wet corner repeats in the same place every wet season, and virus moves around. Only a sketch from last year tells you which one you are looking at.
Smut, ergot and things you should not eat
Loose smut is unmistakable and slightly shocking the first time. Instead of a panicle of green spikelets, the plant pushes out a black, sooty, powdery mass that disintegrates in the wind. The fungus has been inside the plant since the seed germinated and there is nothing to be done for that plant. What you can do is stop it spreading: cut the affected shoots into a bag before the dust starts moving, and do not save seed from a bed that had smut in it, because that is exactly how it travels to next year.
Ergot is rarer on oats than on rye but is worth knowing because it is the one genuine safety issue in home grown grain. It replaces individual grains with hard, dark purple black bodies noticeably longer than a normal kernel, protruding from the spikelet. They are toxic and they do not cook out. If you find them, pick them out during winnowing and sorting, and if a crop has more than a scattering of them, do not eat that grain at all. Ergot bodies are heavy and they largely survive winnowing, so this is a visual sort rather than something the fan will do for you.
Beyond that, a garden oat bed is a remarkably trouble free thing. Slugs will graze seedlings in a wet autumn sowing, and mice and voles will work a bed near a hedge, both of which are dealt with in the usual ways. The most reliable losses are not disease at all: they are birds between heading and harvest, and grain shattered onto the soil because the crop was cut a week too late. Fixing those two costs you more grain than any spray ever would.
Rotation and what to do with a failed bed
Do not follow oats with oats, and do not follow oats with wheat or barley either, because crown rot and take all style root diseases build in the soil across any run of cereals. Two or three years of something else in that bed, ideally including a legume, resets it. This is easy advice to follow when the oats were a cover crop, since you were planting something else there anyway, and harder when you have found a bed that suits them.
A stand that has clearly failed, whether from virus, from rot or from a lodging that never recovered, is still worth something. Cut it green before it sets any seed and either compost it or turn it in as a green manure. Oat top growth is soft, nitrogen rich when it is young, and breaks down quickly. There is no reason to leave a failed bed of grain standing to look at all summer.
Keep a short note each season of what showed up, where in the bed, and roughly when. Rust that arrived at flowering and rust that arrived at hard dough are entirely different events for your yield, and a year later you will remember only that there was rust. The date, and the growth stage the crop was at when it appeared, is the part that lets you decide whether to sow earlier next season.