Aster yellows

What you are looking at

A flower that has come out green. The petals are there in outline but they are leaf-like, stiff and the colour of foliage, and the flower will not set seed. On a coneflower the centre often erupts into a tuft of small green shoots, which is the version most people photograph.

The plant around it is stiff and upright, with new leaves narrow and yellow while the older ones stay green. Sometimes a point that should have carried one flower carries a dense broom of thin shoots instead.

In carrots the signs are underground as well. The root grows a beard of fine hairy rootlets, tastes bitter, and the tops go red and twisted.

What separates it from most things is the pattern in the bed. Single plants are affected here and there while the ones beside them are untouched, because each infected plant was visited by a leafhopper rather than infected by its neighbour.

Something between a bacterium and nothing at all

A phytoplasma is a bacterium that has lost its cell wall and most of its genome. It cannot live outside a host, it cannot be grown in a laboratory dish the way ordinary bacteria can, and it lives in the phloem, the tissue that carries sugars around the plant.

From there it interferes with the plant’s own developmental signalling. The green flowers have a name, phyllody, and they are the plant building leaves where the genetic instructions for petals should have run. Virescence is the related symptom where the petals form but come out green. Neither is damage in the ordinary sense. The plant is being redirected.

The effect on the plant is sterility, and the effect on the phytoplasma is that a sterile plant keeps growing leafy tissue full of phloem for longer, which is where it lives and what the leafhoppers feed on.

Leafhoppers spread it

Leafhoppers acquire the phytoplasma by feeding on an infected plant, carry it for a period of one to three weeks while it multiplies inside them, and then transmit it for the rest of their lives.

An insect that feeds on your diseased coneflower today cannot infect anything for a fortnight, and that fortnight is the window in which pulling the plant out does some good.

The phytoplasma does not live in soil, is not carried on tools, and is not spread by seed. It survives the winter in the leafhoppers themselves and in perennial weeds that carry it without dying, and dandelion, plantain, thistle, wild carrot and chicory are all on that list. Clearing the rough margin around a plot is therefore a real control and digging over the bed is not.

In North America the picture has one more layer. The aster leafhopper does not overwinter in the north and migrates up from Texas and Mexico each spring, so how bad a season will be depends on conditions far to the south, and extension services issue forecasts rather than reports.

What to do about it

Pull out an infected plant as soon as you recognise it, bag it and take it off the plot.

Look at the flowers rather than the leaves when you walk the bed, since green petals are the sign that arrives first.

Clear dandelion, thistle, plantain and wild carrot from the margins.

Mesh carrots, celery and lettuce from sowing, and keep the mesh on.

Discard divisions, offsets and stored roots from an infected perennial, and keep the seed if you want it.

Spread the susceptible daisies through a mixed border instead of massing them.

Replant the same spot without hesitation, since nothing is left in the soil.

What it gets mistaken for

Reversion sends one strong green shoot from a variegated plant, and the shoot is otherwise normal.

Gall mites swell buds and distort leaves and leave the flower structure alone.

Hormone weedkiller twists everything downwind of it in a gradient rather than picking out single plants.

Mosaic viruses mottle the leaves and leave the flowers normal.

A plant that never makes buds is short of light, cold or feeding.

Nutrient shortages yellow whole leaves evenly across a bed and correct when you feed.