This weekend I have been learning about viticulture. I have three grapevines, grown largely as ornamentals (providing dappled shade and screening for seating areas etc), but all of which have also produced lots of bunches of grapes, which we believe to be table grapes rather than wine grapes.
However, one of the three vines, growing over a small patio at the front of the house, has started to look quite unhealthy in recent weeks. A lot of the leaves have turned brown and crispy and started to fall, making the patio look like it is autumn, and the bunches of grapes are starting to dry up and shrivel.
On closer inspection, there seem to be two things wrong with the leaves.
Firstly, some have bumps on the surface like large blisters (maybe 5-15 mm diameter), and on the reverse, something brown nestled in the hollow, looking a bit like a scale insect. This is something known as grapeleaf blister mite (or erineum mite). Fortunately, apart from looking ugly and rather worrying, it does not cause any real harm to the plant or the crop.
Grapeleaf blister mite
The second thing is the browning of the leaves. While some leaves are brown at the edges, and others have completely dried up and either fallen or remained stuck on the vine, there are also brown spots on some of the leaves, with a darker margin.
This (along with the mummified grapes) is what lead me to the diagnosis of black rot. Black rot is one of the most common problems encountered with grapevines. It is a fungal infection, set off by warm and humid conditions, with spores overwintering in the dried up fruits, and in black lesions on the canes and tendrils.
Lesions on cane
Apparently the brown spots appear first on the leaves, then the whole leaf turns brown and dries out, then the fungus reaches the grapes, first causing a sunken rotten-looking spot on the grape, then shrivelling and shrinking them into blue-black ‘mummies’, which remain on attached to the cluster.
The way to tackle black rot is mostly good plant husbandry and hygiene – clearing up the fallen fruits and leaves and cutting out infected material to prevent overwintering spores – and spraying with an appropriate fungicide in the spring when the vines start to grow again.
I decided to start right away, and have removed all the bunches of grapes from the vine (all were infected to some degree), and any particularly badly infected leaves and canes, whilst leaving enough foliage to keep the vine alive and provide some shade.
This has also allowed me to see the structure of the vine, which is actually at least two vines, growing up from either end of the patio and trained over wires across the top. There are lots of thick, gnarled canes twisting up at either end, but removal of a lot of material has enabled me to see that more than half of this old wood is completely dead, and not contributing anything to the plant except a probable overwintering site for the fungal spores!
I think I will wait until the dormant season before I do anything more, but these old canes will come right out, allowing me to remove a lot more infected material, and also to start training the vines properly. From what I have read, you can’t prune a grapevine too hard, and even if you cut the trunk hard back (which I may do) it will sprout afresh and be healthier for it. I’m looking forward to getting ruthless with it this winter!