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Date last updated: 
01 August 2008

The White Rust Story By Ivor Mace 

Ivor Mace

Background
I first heard of white rust in the early 1980's when Bert Boon had been speaking to someone up at chrysanthemum HQ. I believe the Ministry of Agriculture had warned the Society that this serious disease had appeared on commercial nurseries in the UK. Short articles and photographs soon followed in Society publications, but they were brief and sketchy, no one in the western world knew much about it. It was as though it was an alien descending on us from Mars.

Notifiable Disease
What we did know was that if we suspected that we had White Rust on any of our plants we were to notify the ministry of agriculture. If their inspector confirmed an out break we would have to destroy all our plants and start again. This sent shockwaves through the chrysanthemum fraternity. In 1983 a friend of mine Tony Cannon got white rust in his plants. Tony lives in Ogmore Vale about 10 miles from me. He grew about 50 pots of late flowering plants, and mainly showed in his local society show. He was devastated. I only know the year that the white rust out break hit him because he liked growing charms and specimen plants. At that time I worked in my local parks dept nursery and we grew charms and used them in displays. When the phone call came from Tony saying his stock had been destroyed I offered him the spare plants we had at the parks nursery. We had all the well known varieties of charms but we had grown some from seed, from a packet of seed from Suttons Seeds. Most of these were inferior to the named varieties except for a nice cream coloured one we had selected. I passed on a few of these to Tony and he got it registered later that year and named it Ogmore Vale. Personally I still hadn't seen the disease at first hand, however that soon followed when Brian James thought he had a possible outbreak and several of us visited his greenhouse on the day of our Group show. Our conclusion was that he did indeed have it on his plants, at least it looked like the photos we had seen in the NCS article. Brian also had to destroy all his plants.

After a period The Ministry or DEFRA or what ever they are called now, relaxed the rule and growers were allowed to treat their plants with fungicide applications, and the destruction of plants would be unnecessary.

Job change and Sprays trials
When I changed my Job from the Parks Dept to Pencoed College I brought stock of spray chrysanthemums with me because I knew my boss was keen to develop trial grounds on a small scale like he had encountered at Wisley during his training. The stock of spray chrysanthemums I took with me was free of white rust and I never encountered any until about 3 years later which would be about 1992. Eventually we decided that there was no point in growing the established early spray varieties over and over again because everyone who was interested in chrysanths knew all about them. 

At that time I was on the executive committee of the National Chrysanthemum Society (trustee) and knew most of the growers and nurserymen, they also knew me quite well. Therefore it was quite easy to ask them if they would supply their new varieties for us to trial them so that growers in South Wales could see them at first hand. Several nurseries responded - Riley's, Southway's, Walker's, Don Horn and other growers who had new early spray varieties from time to time. We were able to get enough new ones and by propagating some cuttings in subsequent years, we could then give them a two or three year trial. 

I can categorically say White Rust never broke out on the trial grounds. However it did arrive from one nursery on un-rooted cuttings. These arrived on a Wednesday morning. I duly inserted the cuttings into plantpak 24 modular cells and placed the trays on the propagation unit. The unit was an electric soil warming cable with a mist unit fitted, this was because I only worked from 8-30am to 4-30pm Monday to Friday. Other staff covered weekends but they wouldn't have enough time to look after plants on the propagator. 

Every leaf had white rust!
I arrived in work on the Monday morning to find every leaf had at least 20 White Rust spots on them. The mist unit proved to be an ideal incubator for White Rust spores, but believe me there must have been a hell of a lot of spores sitting on the young leaves ready to germinate. My mind ran riot as I tried to imagine what the stock plants looked like that those cutting had come off. How many people had had cuttings from that source? How far across the UK had this strain of White Rust been spread? Needless to say I bagged them up and got them burned in the college coal boiler.

Nothing else affected
At the same time the plants from the other nurseries were in the cold frame just a few feet away but the other side of the glass. I had constructed cold frames running the length of the greenhouse, backing onto the glasshouse on either side, these had Dutch Lights in place. White rust never appeared on any of the other plants at all, neither in the cold frame nor on the trial beds which were about 200 yards away. When I got home I took a shower and put all my cloths into the washing machine because we were led to believe it could be carried on our clothes and I didn't want it spreading on to my beloved Japs which were in my garden at home. 

Don't believe all that you read
The mist unit was turned off for the summer period and in October I decided I would need more stock of a few varieties of Japs that I only grew in small quantities. I brought basal shoots with me to work and inserted them into Plantpak 24 modular cells, I wetted the sand in the propagator and turned the mist unit on, and lo and behold after about 2 weeks just as the cuttings began to root I noticed White Rust on a few cuttings. We were led to believe White Rust would only live for a very short period unless it was active on chrysanthemums. That was my first lesson learned don't believe all you read. From now on I thought to myself I am just as much an expert as the people who were advising us. Seeing is believing, the spores had lived on the dry sand from April to October and when conditions were right they germinated.

Two types of spores
More recently I read about two types of spores, resting spores called Telospores which they say can survive for up to 8 weeks in dry conditions, well I can more or less prove the only white rust we had that year was on those infected cuttings and no other chrysanths were near the dry propagator for 6 months, so I dispute the scientists claim, they also claim that after 3hours of 95-100% relative humidity they develop into Basidospores and these would need a film of water for 2 hours at 17C to germinate. This fits in perfectly with the situation on the mist unit. May I say that the plants at home, where I took the cuttings that got white rust on the propagator never developed any white rust at all, this proved to me that it had survived dry on the mist bench for 6 months only to become active again when conditions were favourable.

At that time Propiconazole was the only recommended chemical for control of White Rust, the amateur could buy it as Tumble Blight (now withdrawn) and the professional grower could get it as Tilt or Radar. I had a litre of Tilt in the college pesticide store just in case White Rust appeared; I mixed it up at the manufacturer's recommendations and soaked the sand in the propagation case, the floor, the glass and anything else that was lying around. In a few days when everything had dried off I brought some more cuttings from home and inserted them and turned the mist unit on. They rooted and no white rust ever appeared on them.

The scientists say that Telospores only live for 8 weeks under dry conditions (which I dispute) but they die much quicker under moist conditions which I believe because if it was warm enough they would germinate into Basidospores. These are more fragile and would need to germinate pretty soon or die. I must say I have never had a re-infection on the standing ground in the following season or future seasons so this confirms this theory.

The Two occasions that I have had white rust at home on my own plants.
The first occasion was around 1997, I might be a year or two out, but it was around this time. As many of you know my lifelong chrysanth pal is Dennis Fletcher. Dennis helps out on a Wednesday and Sunday at our Gardening Club store shed situated at the end of our road. There are only 4 houses there, I live in number 2 and the shed is next to number 1, so you can see it is only a matter of yards away. A gentleman called at the shed and unfortunately Dennis hadn't arrived. 

He had a few chrysanthemum leaves covered with white spots. The other members in the shed didn't know what the problem was, so they sent him up to me, carrying these leaves, which were smothered in White Rust at the active stage. When he knocked my front door he was only a matter of yards from the top rows of my chrysanths, as most of them are stood out in rows on a third of my front garden. Although I got those leaves quickly into my coal boiler and burned them about 7-10 days later I got a few plants with some White Rust spots. 

The weather was too inclement to be able to spray so I got an artists paint brush and dabbed the spots both sides of the leaf with Propiconazole. I reckon I was very fortunate in that I half expected it to break out on my plants and I was looking out for it. Being an initial outbreak there were relatively few plants involved and only a few spots. I reckon I was lucky to spot them all and get them before they could release spores to go on to cause a second generation outbreak. I kept a daily look out for the rest of the season and cut the basal shoots back hard after flowering and sprayed 3 times before the new shots had emerged. The following season I was clear again. 

In the 1992 season I went up to St Albans to the miniature rose show and travelled on to Peterborough to see Andy Wickham. The following morning we went to visit Bill Croft. Bill had sprayed his plants the week before with Amistar (Azoxystrobin) Bill was extolling the virtues of Amistar saying that unlike Propiconazole which seems to make the plants brittle, the plants seemed to look better after they had been sprayed with Amistar. When I got home I checked in the UK Pesticide Guide and Amistar had indeed got off label approval for white rust on chrysanthemums. I though it would be a good idea to get some as insurance, just in case I got another out break of white rust.

Learning a valuable lesson
The following season a raiser gave me a few plants of a novelty in order that I could try to get a vase or two to show this new variety to other growers at the shows I go to. I learned a valuable lesson; I never thought that these few cuttings could cause such problems. I didn't take any precautions and I paid the price. 

I never noticed any White Rust on these plants until late June and by then it had spread to the Gigantics and Harry Gee's which I now know are very susceptible varieties. I sprayed with Amistar and the spots seemed to have been killed, only for new spots to break out again on the new growth. I didn't want to use Radar (Propiconazole) because I didn't want to stunt the plants, so I sprayed with Amistar again and once again the spots appeared dead, but they broke out once more. 

We were having quite a lot of rain which was creating ideal conditions for the germination of White Rust spores and it was preventing me from spraying. I decided to pull the covers over so that at least I could spray as it would be drier under the cover. When I had been spraying the chrysanths I finished off the remaining insecticide/fungicide on the 24 rose bushes that are growing in a bed between the chrysanths and the house. These roses were the healthiest of all the bushes I had, and they were the only ones that had had Amistar. 

Off Label approval
At that time I was having problems with Downy Mildew and Blackspot on my roses and just like chrysanths, roses are not a commercial crop of any significance in the UK or worldwide in comparison to wheat, potatoes, brassica's and so on. Therefore there are no fungicides developed specifically for roses or chrysanths. When problems like Downy Mildew on roses or White Rust on chrysanths cause serious losses to commercial growers the government agencies test a range of fungicides that are developed for other diseases on other crops and if they control the disease without harming the roses or chrysanths they give them off label approval. 

Two fungicides used to control Downy Mildew on roses are Folio Gold (chlorothalonil +metalaxyl-M) and Aliette (fosetyl-aluminium). In a desperate attempt to find another fungicide to alternate with Amistar I tried Folio Gold, after all if Amistar controlled Downy Mildew and Blackspot on roses, (in fact I have subsequently found Amistar controls most rose problems better that the approved products) why couldn't Folio Gold control White Rust, and after all I was getting pretty desperate by now. I found it did kill white rust without damaging the plants. By now the plants had mature leaves at the top of the plants and the buds were there. 

However a few years later I used Folio Gold on stools with young cuttings that were starting to grow and it affected them badly especially Duke of Kent. Anyway I limped along for the rest of that season and decided that once again the best remedy would be to cut back hard after flowering and begin spraying. I used Folio Gold (chlorothalonil +metalaxyl-M) followed by Radar (propiconazole) followed by Amistar (azoxystrobin) with 10 days between each spray. In the spring the cuttings appeared clean right up to when the plants went into the cold frames in April and by about the third week in April I noticed a few spots of White Rust on one or two plants of Gigantic and Harry Gee, the same old suspects again. 

My heart sunk because I thought I had got rid of it completely. I had supplied cuttings to other growers and as far as I was concerned the treatment I had given had worked perfectly on the previous occasion. I spoke to Jack Gilbert who was keen to have a few of my spare plants. Jack suggested I turn the plants upside down, let the loose compost drop of the upturned pot and dip the plants. I had a narrow deep bin and dipped the lot and this did get rid of it completely. Since then I have not had a spot of White Rust on my garden.

Trace to a known source
Looking at the problems growers have had with white rust in almost every case the outbreak could be traced to a known source. In more or less every case if someone tells you they have had White Rust, they have either had an outbreak the previous year or they have bought or been given plants from someone. There are growers who have had white rust and believed they have got rid of it only for it to re-emerge the following season. 

Some growers believe it blows in from miles away, scientific evidence says it can carry on the wind short distances. I think in urban areas where neighbours have Korean or Rubelium chrysanthemums hidden behind fences I suppose a grower could get re-infected over and over. However I would never blame another grower for getting White Rust because there for the grace of god go I. After all it caught me out on that one occasion. 

Many a time I have been sat by someone who is blowing their nose with a streaming cold, a few days later I get a cold, I think to myself I bet I caught that from that person the other day. Most likely I did but I can't prove it. So I don't sue them, shoot them, or hit them over the head. It's part of life and we just get on with it.

What lessons have I learned?
I am convinced that the dormant season is the best time to eradicate an outbreak of White Rust; in the active season all you can do is contain it. Think about it, the spore germinates on the upper surface of the leaf and the fungus goes through the leaf only to appear as a pustule on the underside. Spores then release and the disease increases by thousands. One spore can breed thousands. Therefore it is inside the soft tissue of the leaf, after cutting down we have no leaves on the stools. 

With earlies we can soak the stools in fungicide. And then spray several times at 10 day intervals alternatively with Amistar and Bumper (propiconazole is sold as Bumper 250 EC these days). I have searched the UK Pesticide Guide and I have not found a single fungicide with approval for White Rust, however I have found 2 with off label approval, Amistar and Bumper 250 EC. I also dip any cutting that I get in, in Amistar and keep them in a separate greenhouse and spray them several times before they are allowed to go anywhere near my chrysanths. 

It would be nice to know what chemicals were tested on White Rust and which ones didn't work or harmed the chrysanthemum plant. These could then be avoided. After all it is well known that it is a bad policy to use the same fungicide of insecticide over and over because the pest or disease can build up resistance. We badly need alternative chemicals to tackle the disease, but which ones can we use. Some growers have tried a product called Jenton and found it controlled the disease on mature leaves but like Folio Gold on younger growth it has a bad effect on the plants. Officially we are only supposed to use approved chemicals or ones with off label approval, and to use commercial products we should have a pesticide certificate. 

Storage of chemicals
Our pesticides should be stored frost free, out of extreme heat, be locked away and the storage area should be able contain a leaky bottle so that it doesn't get into the environment. An old fridge laying flat would do the job. It's insulated and leak proof. An old deep freeze is even better because it can be locked.

Eradication is possible
Many growers have learned to live with white rust. I believe it is possible to eradicate it altogether, if you think about it I have only had White Rust in my garden for 3 seasons out of the last 11. I have got rid of it completely on 2 of those occasions, it has only returned once. We could eradicate it altogether by adopting this approach. 

There is a word in Welsh called ucha fe. When I have asked about its meaning everyone tells me it's un-translatable. I know when I was a kid in school the teacher caught me picking my nose and said ucha fe. Another time I stepped in dogs excreta and unknown to me brought it into the classroom and it stunk, again he said ucha fe. You get the idea of what this means now I'm sure. When a local grower got White Rust on his plants the other growers said ucha fe as though he had caught head lice, or fell in a sewerage pit. 

Well I think that stigma has gone now, other growers are more sympathetic because few have escaped it completely over the years. Nevertheless I don't think we should have the attitude 'we have to live with it' rather we should be trying to eradicate it completely.

Introduction
When you have a long association with a plant genus you get to know almost everything about that plant and its problems. However occasionally something comes up that has never affected that genus before, sometimes it is something that can be easily managed, but occasionally it is something more serious and it troubles us for many years. White rust certainly fits into that category.

 

Copyright 2006 Paul Barlow.