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Diseases of Snowdrops

12/03/2011

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Two quite different fungi are potentially serious problems on cultivated snowdrops. Below we present our understanding of what they are, how to tell them apart and what can be done about them.

Many fungi have relatively complex growth cycles comprising a number of different stages. Each stage produces a different kind of spore with differing infectivity. A full understanding of the fungus life cycle will make it easier to identify the stages and times where it is easier to "attack"; because by curing the disease on the plant we are disrupting the infection cycle of the fungus. Detailed information on the life cycle of these two fungi does not seem to be in the public domain and I would be extremely interested to hear from anyone with knowledge of sources of such information.

Grey mould caused by botrytis galanthina.

Botrytis are a range of grey sooty moulds familiar as the grey, fluffy mould that causes soft rot of fruit in the domestic fruit bowl and sometimes causes serious damage to tomato plants in greenhouses. There are a number of botrytis species; each relatively specific in the plants that they attack. The type that attacks snowdrops is botrytis galanthina.

Emerging snowdrop shoots appear greyish and mouldy and soon come to a standstill compared to other emerging shoots round about. If you do nothing they slowly collapse and disappear leaving a small grey fluffy hemisphere of fungal material where the emerging snowdrop shoot once was. Surrounding, uninfected snowdrops will usually continue growing normally. If, however, you gently tug the emerging infected shoot when you first spot it; it will come away in your hand and come out of the ground without the bulb or the roots. The lower portion of the shoot will be more seriously infected than the leaf tips and already be slimy and disintegrating. The fungus attacks the new snowdrop shoot as it emerges from the bulb, presumably from fungal material in the soil. Once the fungus has overcome the plants outer defences, as it were, it rapidly spreads throughout the bulb killing it and all daughter bulbs that have a living tissue connection. Adjacent bulbs often grow normally indicating that the fungus is unsuccessful in penetrating their cuticle and destroying them.

Botrytis is a necrotrophic pathogen meaning that its' strategy is to infect and destroy a host; relying for long term survival as a species on continually finding new hosts to infect and destroy. This is in contrast to a parasite which usually doesn't kill the host outright. This is an important distinction and is a point that we will return to in the context of the other fungus disease, stagonospora.

Botrytis apparently leaves resting spores in the soil in the vicinity of a destroyed bulb which then invade emerging shoots in succeeding seasons. Treatment is not easy. By the time the symptoms are obvious, the disease is probably too far advanced to save the infected bulb. Published information suggests that early treatment with benomyl or thiophanate-methyl fungicides can sometimes be effective. If the disease is in the middle of a clump of something valuable, then it would be prudent to lift the clump, destroy the infected bulbs and replant the others in fresh soil in a different location after a preventative drench with a fungicide. Where bulbs are naturalised in grass treatment would be much more problematical. Published information differs on whether the disease is more widespread in mild or hard winters. Until these last two hard winters (2009 - 2011), I thought that the disease did not occur this far north. However,in spring 2010 the disease was prevalent in the Cruickshank botanical garden in Aberdeen and destroyed large numbers of Freds Giant. Spring 2011 has seen a few bulbs infected in our propogation beds for the first time. This article has been written partly as a result of the numerous enquiries that I have received on the topic (spring 2011) which would suggest that it has been a bad year for the disease throughout the UK.

 

Narcissus leaf scorch caused by stagonospora curtisii.

This fungus disease has only recently been recognised as a serious disease of snowdrops. It has undoubtedly been around for a long time but because there is virtually no sign that a disease is in progress it has, I suspect, gone unnoticed until recently. Indeed, because plants just disappear without trace over one or two seasons, it has probably been the culprit where people thought that the plant in question was finicky. Both Merlin and Alleni have been described as easy by some growers and difficult by others: perhaps this disease has been the reason in many cases.

Our first experience was in a clump of Benhall Beauty. On casual inspection, the very tips of the leaves seemed damaged and we assumed that someone had strayed from the path and trod on the emerging tips. The performance of the clump was, with hindsight, somewhat poorly. Next spring only one bulb came up and didn't flower; the next spring they were all gone.

Infected plants will display very small pin prick sized lesions near the tips of the emerging leaves. These will be surrounded by a small area of reddish discolouration. Where the lesion is on the edge of the leaf, the leaf tip will have a distinctive bend towards that side. Occasionally, the lesions will occur on the emerging scape or spathe. No other symptoms are visible.

I assume that these infection sites produce spores that can  be wind dispersed to spread the disease to other plants. I further assume that spores are washed by rain down the leaves and attack the leaf bases of the leaves that produced them. This is all inference on my part but would fit within the life cycle patterns of related fungi.

It is this infection of the leaf bases that does the damage. Infected bulbs have much of their structure missing as if eroded or eaten away. Infected bulbs often give off a strong, not-unpleasant gingery smell. As the bulb has lost much of its' structure and thus food reserves, subsequent growth is likely to be greatly impaired. A repeat infection cycle in the next growing season will destroy the bulb entirely.

Unfortunately, treatment requires fungicides unavailable to the home gardener. All our bulbs are harvested, cleaned and replanted annually. When out of the ground they are dipped in a fungicidal wash. They are also sprayed once; on a warm day sometime after leaf emergence and before flower formation. This regime has been successful in eradicating the disease as a problem. Published information suggests fungicides based on carbendazim and we use a proprietary product called Bavistin. We found Dithane 945 to be completely inneffectual.

The disease is widespread on many kinds of Ammaryllids, where it seems to co-exist as a nuisance, almost like a parasite. Why it should be so destructive on snowdrops is unclear; perhaps their smaller bulb structure with far fewer layers plays a part.

If anyone knows of published sources of detailed information on this disease, please send an email to the site.