Botrytis brings an end to the tomato crop

Grey mould has quickly overcome this tomato foliage

Grey mould, caused by Botrytis cinerea, has struck both the glasshouse and polytunnel tomato crops. Spores of this fungal disease are generally present in the environment and typically infect dead or dying plant material. In theory, grey mould could cause problems at any time of year, but in order to develop significantly, a certain amount of moisture is necessary along with cool temperatures. Under cool and humid conditions, though, the spread can be very rapid indeed, resulting in extensive damage to both plant and fruit. The high levels of humidity brought on by the recent rains along with cooler autumn conditions have allowed grey mould to develop amongst the tomato vines, so action was taken to rescue as much as possible of the remaining crop. This is a bit of a shame, as we are probably still several weeks away from the first frost, and are forecast a few more days of good sunny weather.

Preventative actions focus primarily on good ventilation. Our polytunnel has full length insect proof vents down both sides, as well as ventilation on the front door, which I often leave open anyway, and the glasshouses have double sliding doors and automatic roof vents and louvre vents. For most of the year, there is good ventilation and, so long as the weather is reasonable, we have little trouble with grey mould. Conditions could perhaps be improved further by using fans to keep the air moving. In particular, addressing the difference between air temperature and plant temperature that could cause condensation to form on the plant matter might be beneficial; without such moisture, the moulds cannot readily develop and spread.

Overcrowding is also worth bearing in mind as a contributory factor. I plant tomatoes in a single row, but rather close together, at 1’ spacing. Typical recommendations for spacing are around 2’ to 3’, but the closer spacing works very well for me, provided that side shoots are regularly removed and new growth tied in, otherwise it can turn quickly into a jungle of growth. It also requires attention to providing adequate soil fertility and water. Such close spacings are, though, somewhat detrimental to good air movement, but this can be improved by removal of older leaves. Indeed, regardless of spacing, it is common practice to remove all of the leaves below the currently ripening truss as well as any that are yellowed or in poor condition. This increases air flow around the plants, helping to prevent conditions forming that are suitable for mould.

High levels of humidity, or condensation, forming either directly on plant matter or dripping onto the plants, are needed for moulds to develop. However, they can also gain a foothold by infecting wounds, such as pruning cuts, or other damaged tissues. Naturally, when pinching out side shoots, one is regularly providing potential entry points through which diseases can infect the plants.

Even with good ventilation, the weather has been so miserable and humid over the past couple of weeks that grey mould has become quite firmly established. Appropriate remedial action would rather depend on the time of year. Early in the season, ruthless removal of all infected material would be in order, along with removal of such leaves as practical without causing too much stress to the vines, and any other measures that may be taken to improve air flow. Good hygiene, including the removal of all dead plant matter and fallen fruits, is important. I must confess that I often fail miserably on that point; with too many other projects on the go, the glasshouses and polytunnel readily end up in a bit of a mess.

Suggestions may be found for various natural fungicides that may be of some use. I have tried a dilution of milk for the control of the powdery mildew that often hits the summer squash plants. In my limited experience, although not eliminating the problem entirely, it does seem to provide reasonable control for a week or two, after which another treatment is needed. It may be that a milk solution could also be a useful tool in tackling Botrytis, and if I find the problem occurring earlier in the season, I may well try it. I have also read that mixtures with bicarbonate of soda, typically combined with a natural oil and soft soap, can help prevent the development of fungal diseases, supposedly by making plant surfaces more alkaline, inhibiting germination, but I have not yet tried this myself. There are, of course, the copper and sulphur based fungicides, such as the famous Bordeaux Mixture, which is on the margins of acceptability for organic use. I do have some, but have not yet resorted to using any.

At this time of year, there is little point in fighting what would no doubt be a losing battle to control the spread of grey mould, when there are but a few weeks remaining before the first frosts come along to put a definite end to the season. Instead, we harvested all of the undamaged fruit, sorting the ripe from the unripe and almost ripe. The former we turned into another eight litres or so of concentrated tomato sauce, simmering them on the hob until reduced by at least one third. The latter we have spread over several layers of newspaper on shelving in one of the outbuildings. We could bring indoors a handful of nearly ripe tomatoes, and even a few fully green specimens, and ripen them in the fruit bowl, where they benefit from the ethylene given off by other ripening fruit, as well as the warmth. My grandmother used to keep them in brown paper, in a drawer, often with a banana to hasten ripening. However, we were rather curious to see how well they would store in a fairly dry and airy outbuilding, as we have never tried that before.

I have read recommendations to wrap each tomato in newspaper, as is sometimes done with apples, but that is rather impractical for the quantity of fruits that we have remaining. Individual treatment like that might be good for a small selection of large fruits. Some also recommend cutting the vines at the end of the season and hanging upside down; that would perhaps be a good approach if the vines were free from disease. Instead, we simply spread the tomatoes out as best we could on three shelves, most with some vine attached. Ideally, they would be in a single layer and not be touching, in order to prevent the spread of any moulds that develop whilst in store. However, we were pressed for shelf space, so they are strewn with a little less care than is ideal. It will be interesting, though, to see how well they store, in terms of losses to rots, how long they keep before either rotting or ripening, and how those that do ripen actually taste. If it happens that ripening is fairly successful, and extended over the course of a few weeks, we would be rather inclined to do the same next year.

Of course, green tomatoes – or, perhaps I should say unripe tomatoes, as we have some that are green when ripe – are still rather useful in the kitchen. Although not so common here, fried green tomatoes are quite a speciality in some locales. They can be roasted with other vegetables, incorporated into sauces, and are an excellent ingredient in a range of pickles and chutneys; I am hoping to make some time to prepare a few jars while we still have them.

One comment

  1. I too have the Botrytis problem and your excellent blog has answered just about all the question I wanted to ask, so thank you.
    I think it is too late so try remedies now but will take note of those and the advice on overcrowding next year.

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