Introduction Each growing year brings its own set of problems and 2007 was no exception. After the warm April we experienced, who could have forecast the wet summer to follow which resulted in such devastation and widespread rust? The situation being compounded by there now being many strains of this disease. An outbreak presents the grower with a demoralising situation as control seems difficult if not impossible. I experienced rust in my first year of growing, about 1996 with 100 plants being grown for garden decoration; I did not grow for exhibition at that time.

All seemed well until early October when I noticed a few white spots, closer inspection revealed all my plants were totally plastered, I have yet to see a heavier infestation. At the time I did not know what was wrong - I soon learnt. Most plants were destroyed, about ten were retained and treated stock from these stools is still being grown to this day with no sign of rust. About this time I joined High Wycombe Chrysanthemum Society and it became obvious I was not alone with this problem. I learnt rust was a particular problem in our area but that there was an answer. After speaking to a number of members it seemed to me that white rust was similar to potato blight, possible to prevent but virtually impossible to eradicate from growing plants.

I grow about five pole of potatoes on part of my allotment on a 65 plot site. Upon the site we are blessed with just about every pest and disease in creation, anticipation is essential and one thing is certain, potato blight will arrive by the third week in July. For many years I have found two or three applications of Dithane 945 at ten day intervals from the end of June onwards will keep the blight at bay even though nearby plots are afflicted. So when some of the more experienced growers in High Wycombe Society suggested preventative techniques could be applied to avoid chrysanthemum rust their advice immediately rang a chord.

I have used this advice to suit my own growing conditions and it is outlined below. None of it is new, however it may provide food for thought and a few reminders for future years.

Firstly we need to appreciate that with our climate an outbreak of rust can occur at any time from about March until November.

If rust occurred on or near to a chrysanth plot one year it is quite likely it will find a nearby host plant to overwinter, great care will then be necessary to prevent it returning the following year.

How does it spread?
In addition to the above the disease can be spread in a number of other ways including:-

1. Windborne - probably the most usual source of the spread of infection as a single spore can travel great distances -I suspect several miles on a prevailing wind. 
2. Via new stock from infected sources. 
3. Via insect movement. 
4. By human movement between plots. 
5. By taking home flowers from a show that may have been staged next to infected exhibits. 
6. By using waste material from our own plants as a host. 
7. By the use of mother plants where the parent cutting has been taken from infected stock.

Those with more scientific knowledge may disagree with some of the above - I would prefer to err on the side of caution.

Germination conditions A single spore of rust needs the combination of a chrysanth leaf, a film of moisture and a warm temperature to germinate. Moisture can be provided in several ways including rain, dew and overhead spraying by ourselves. Once a spore has germinated the pustule below will, in about two weeks release many thousands of further spores amongst our plants resulting in a major outbreak. Unfortunately many of the chemical sprays that have been tried only seem to suppress the disease rather than eradicate it and cuttings taken from such stock often show rust very early in their growth. Excessive use of some chemical sprays also has a debilitating affect on plants. Seeing as we tend to grow our plants in long rows [windbreaks?] the odds seem stacked against us, so what to do? Left: underside of infected leaf

My procedure
The procedure I adopt can be summarised as follows:-

1) Start the year with clean stock that has been warm water treated. 
2) Be ruthless if rust appears on imported stock. 
3) Apply preventative fungicide regularly and increase frequency at times of high risk. 
4) Prevent germination of rust spores by keeping foliage dry as far as possible. 
5) Maintain constant vigilance so that if any spores get through our defence and germinate they are removed before they can infect the whole plant and multiply.

By way of a reminder I will expand on the above as follows:-

Initially we need to start with clean stock and therefore warm water treatment is essential. This has proved an effective method of eradicating rust from infected stools and even clean stock is best treated each year as a precaution, it should not however be carried out too early . Treatment also needs to coincide with a complete break in the growing cycle, a period of about two weeks when there is no green leaf material on either the stools, the plot or in the greenhouse, this is not easy especially if both earlies and lates are grown. If stools of early plants are treated too soon, say in October, green cutting material may quickly re-grow and be cross infected if the nearby lates are carrying rust. This may seem obvious however it is common for rust to carry over from year to year in this way. If mother plants are being grown at this time of the year great care must be taken to ensure they are taken from clean stock and not carrying rust from one year to the next. There could also be a host plant growing nearby of which we are not aware and our climatic conditions during October and November in recent years have certainly been conducive to the spread and germination of rust. Mid to late November or even December is therefore the safest time for warm water treatment. All my stock has been treated annually for the last ten years as a precaution.

Warm water treatment
Prior to treatment plants will have been rogued, labelled and basal growth kept trimmed. Stools are lifted from the plot or cut out of pots, soil shaken off, washed, basal growth and roots cut back so the stool resembles a paint brush and all green leaf material is removed. A reliable timer and accurate thermometer are required. Within our Society a wide range of water containers are used ranging from old saucepans, converted tea urns, an old clothes boiler and the like up to very sophisticated laboratory equipment. Any suitable receptacle can be used so long as it does the job and is safe.

The water temperatures of 115F or 46C and immersion time of five minutes are well known. I push the temperature up to 120F as when immersing cold stools the temperature tends to drop to just above 115F. If starting with water at 115F the temperature will drop below this threshold and the treatment will probably be ineffective. A bit of experimentation with some surplus stools may be necessary to ensure the requisite temperature is maintained. After exactly five minutes in warm water the stools are plunged into cold water to stop them cooking. It has been known for growers to lose their entire stock by forgetting this bit. Several cold water containers are normally necessary; when cool the stools are laid out to drain. Some growers box up immediately, others prefer to let their stools dry naturally for a couple of days, I prefer the later as this enables me to hold back those cultivars from which I want end of February or March cuttings. When boxing up sterilised compost is used, the stools then go into cold fames or the greenhouse and are kept cool. Stools that have been treated are a little more tender and should be kept cold but frost free, fleece and bubble insulation are useful here. I find most cultivars love this treatment but some are not so keen. With me, those less keen are Enbee Weddings, Ginger Nut, Lorna Wood and Minstrel Boy, these are still treated however extra stools are retained as a few losses are possible. There may be a few other cultivars that I have not grown before which are adversely affected by this treatment, for instance I understand the Gigantics do not like warm water treatment.

At this point it is very important to remember that warm water treatment provides no lasting benefit, it will however ensure stock is free from rust on the day of treatment, a bit like an MOT Certificate, valid for the day of issue. From now on preventative measures and a good standard of horticultural hygiene are necessary throughout the growing cycle.

New stock and imported cuttings If acquiring new stock I prefer to obtain stools that can be treated as this is the safest option. Otherwise reliable rust free sources are used. I treat all imported cuttings with suspicion for its first six weeks in my possession. Generally I have had no problem although if imported plants have developed rust they have been destroyed immediately before the disease can spread. On this point there can be no compromise, if rust shows on plants at the 3 inch pot stage it is likely the cuttings have been taken from infected stools that have either not been treated or have been inadequately treated during the winter, to persevere with infected plants at this stage is in my view futile. Destruction is the only safe way no matter how expensive or good the stock may have been, more can always be obtained another year, our immediate priority is to keep our other stock clean.

Propagation
Within the confines of the greenhouse our plants should be fairly safe. Whilst on the propagator my cuttings are misted in the morning and during the day are covered with a white split bin liner to reduce transpiration. Once off the propagator I do not mist and aim to keep all foliage dry, water is applied only to the roots. Once established in their 3 inch pots a half strength fungicide containing myclobutanil such as Systhane is applied. With chrysanthemums I adopt the same procedure as for potatoes and other types of plant in that fungicides are used solely as a preventative and need to be applied about two weeks before any sign of infection, particularly at those times of year when the level of risk is high. I find that with most pests and diseases it is not a question of if but when.

Vulnerable in cold frames
When plants go into cold frames they are suddenly more vulnerable. They get more fresh air (wind with the possibility of spores), there can be more water slopping around as watering is not always easy and temperatures are rising. Whilst in the cold frames they get another fungicide which from now on is at normal strength. Water is only given to the roots. If rain is forecast frame tops remain on but are wedged open to ensure adequate ventilation but with the aim of keeping foliage dry.

Once earlies are planted out they immediately get another fungicide as they may be left (ignored) for a couple of weeks while they settle in. During this period we hope for good growing weather such as sunshine and showers which is also good rust weather.

After final potting a similar routine is adopted with the lates as again for a couple of weeks they can be left to themselves (ignored) and during this period should not be left unprotected.

Mid season controls During June and July growth is rapid, leaves are enlarging continually and more are developing daily, over a three week period a plants leaf area may double in size. If fungicide is applied at the start of this period after three weeks we have a plant 50% unprotected and 50% protected by diluted or washed off fungicide depending on rainfall. It is during this period that many outbreaks of rust occur, we have the combination of warm temperatures and showers our plants also have soft vulnerable growth yet to ripen. About this time of year chrysanth plants beyond our own plots and being grown as perennials which have been dormant during the winter will now be growing, any that may be infected with rust could well be about at the point where they are able to release a continuous supply of spores onto the wind.

When this situation occurs it can be a very difficult situation to cope with as there seems to be no respite from infection. This is one of the reasons why rust has been such a problem in some parts of the warmer south for so many years, as the winters get milder the problem has moved further north. Another similarity here with potato blight which often seems to show first on any self setters from tubers that have remained in the ground over winter. If there is a known outbreak of chrysanthemum rust nearby and/or the weather is conducive to the disease I would, during this period spray every ten to fourteen days to ensure my plants had adequate protection.

Frequent spraying may be required
Spraying this frequently last summer was a problem and to keep plants clean required a degree of good luck. In other years when we have experienced long spells of dry weather and if there seems to be no rust around I have gone many weeks without spraying. During such periods weather forecasts are watched closely and if rain is anticipated my plants are given protection. Particular attention is essential during those humid/damp spells (potato blight weather) we often get in July and September. When using fungicide my first choice is Systhane although on occasions Dithane 945 has been used and is probably as effective. Under no circumstances do I give an overhead refreshing spray of water, in my area with the amount of rust that seems to be about this would be sheer folly.

Needless to say all waste material from our plants must be contained, disposed of and not left lying around to become a potential host.

Once the earlies are covered they receive a final fungicide, their foliage is now almost at its growth limit and with overhead protection will remain dry and the risk of rust is reduced. Basal growth is kept trimmed back and when flowers are cut, laterals are reduced to leave a nine inch footstool with virtually no leaves on the plant.

High humidity under early covers Sometimes however if warm temperatures are experienced when the covers are erected; the humidity around plants increases significantly. If any plants are infected at this stage the disease can multiply rapidly unless adequate ventilation is provided. When early plants are under cover we often hear of rust appearing only on basal growth. This probably occurs when there are many spores in the air but those alighting on the upper dry leaves do not germinate. However those alighting lower down find the necessary film of moisture for germination, probably due to careless hand watering, inaccurate spray lines or dampness at soil level, the young softer leaves of basal growth are also more prone to infection.

End of season vulnerability
Plants of early chrysanthemums are also particularly vulnerable to infection after the September shows. Blooms have been cut, covers may have been removed, preventative spraying has probably ceased, unprotected basal growth is growing profusely and the stools are waiting to be lifted. For a few weeks the grower may be having a rest or concentrating on the lates. During the autumn we always seem to get a spell of very damp, humid weather, the occasions when great damage can be done to the lates. At these times our early plants are extremely vulnerable and a small outbreak of rust can easily go unnoticed only to reveal itself on cuttings the following year. It is common for rust to arrive in this way and warm water treatment carried out as a precaution will ensure stools are clean for the following year.

Treatment of lates prior to housing
Prior to lates being housed my greenhouse is thoroughly cleaned and fumigated as usual. Plants get a final insecticide which when dry is followed by their last fungicide, I never mix the two. Contrary to normal practice I house the plants while they are still wet as I prefer the fungicide to dry naturally under cover rather than risk it being washed off by a sudden shower of rain which has happened in the past. The greenhouse door and louvre vent will be open and most plants normally still have bud bags. Damping is unlikely until I close up at night about ten days later when the blooms are more developed and then the dehumidifier is used. About thirty pots go in the greenhouse the remainder go under the early cover and are treated the same way.

Ongoing vigilance is key
An outbreak of rust is likely to commence with the arrival of one or perhaps very few spores which if not noticed and given the right conditions will lead to a major infestation. Vigilance and early detection are therefore crucial. In the 2001 Yearbook, when referring to the number of plants he grew Bill Croft said that he "knew every leaf" I know what he meant. I walk my plants daily and in the ten years since adopting the foregoing have removed five leaves each with a single spot of rust that if left unnoticed could have resulted in a major outbreak. On each occasion a fresh application of fungicide was about due. Removing the odd leaf which has a single spot of rust seems to give some control provided it is a new small localised outbreak caught very early before the whole plant has been infected. 

Cultivar susceptibility When an outbreak occurs its progress across a plot can range from being extremely quick or very slow depending on temperature, wind, and rain. The strain of rust probably affects progress as does how the crop is protected from the weather. Cultivar susceptibility also seems to be a factor. If an outbreak of rust gets hold of a crop things get very difficult, however a situation occurred on one of our members plots eight years ago that provided food for thought. It was mid August and the earlies were covered, rust had appeared on the outer edge just prior to the covers being erected and in this case the disease was now moving very slowly across the plot, some fungicides had been tried but without success.

After a bit of thought and discussion it was decided to try damage limitation, potentially a heart breaking decision bearing in mind most of the blooms were about two thirds developed. All infected plants which comprised about 100, mainly Chessingtons, were carefully cut down and bagged. About 150 plants of other cultivars appeared clean these were retained and sprayed. 

During the next month some of the remaining plants showed very slight rust some none at all, however they provided more than enough flowers and the grower ultimately enjoyed a successful September at the shows, on reflection it was felt the right decision was made. It was most noticeable that some of the older cultivars such as Sussex County, Shirley McMinn and Rosedew seemed to have significant resistance to that particular strain of rust, most of the other cultivars showed partial resistance. 

This experience with some cultivars seeming to be more prone to rust than others raises a number of possibilities. Firstly some of us grow on more than one plot and if so it may be worth considering isolating those cultivars that seem more prone to rust on a plot on their own. From the way in which we noticed rust move across the plot it may be worth planting those most prone to the disease in the north east corner and downwind from the south west (wet) prevailing wind. It may also be worth considering growing less of the cultivars most prone to rust and concentrating on those which are less susceptible. Having started out with clean stock I would expect to apply fungicide between six and eight times in the course of a season, the total number of occasions being dependant on the weather and degree of risk. I consider this a minor task within the overall effort that goes into a growing year. I understand that spraying with Systhane has a hardening effect on plants and will retard flowering, this is a disadvantage I will tolerate if it enables me to control rust.

2007 Summer conditions
Due to the continuous rain last summer it was impossible to keep foliage dry and this resulted in such wide spread rust, it was a good year to be growing permanently under cover. Some growers covered their earlies much sooner last summer, at least one had covers on by the end of June and this proved to be a wise move. Like a number of other growers many of my late plants especially those that need to be grown on the dry side such as Lundy and Majestic were taken back into the greenhouse until late July, those plants left outside had the pots shrouded in polythene to keep some of the rain out.

A few growers are fortunate and escape rust without having to take any precautions whatsoever, probably because there is none in their area, others avoid the disease by only carrying out warm water treatment and being ruthless when necessary with imported stock, in a dry season this might be sufficient, it is during a prolonged wet spell that the greatest problem occurs.

No doubt due to the incessant rain last summer eelworm reappeared, I heard of three cases. Warm water treatment was of course originally developed to eradicate this pest making its use towards the end of 2007 even more relevant than ever.

No quick fix in sight
As growers we are naturally always on the lookout for a solution to a problem, but with rust we may never find the quick fix as nature is constantly throwing up fresh obstacles, mutations being an example. Some fungicides appear to control rust but do not always eradicate the problem from the plant; from our point of view eradication from any stock being used for propagation is essential and this is why warm water treatment at the end of a growing season particularly if rust has been a problem is so crucial. Afterwards, once plants start growing prevention of rust probably needs to be part of our routine and dealt with weekly just as we water, feed and tie in on a regular basis.

In conclusion ..
Some might not agree with some parts of the above however my experience suggests the cultural routine I have outlined has enabled me to keep clear of rust for ten years by using a simple process, readily available fungicides and basic horticultural hygiene, it has also ensured I continue growing.

Having written the foregoing I realise I am now very vulnerable to an outbreak of rust. However I am most grateful to those growers who pointed out how severe the problem could be in our area and then explained how it could be avoided. None of the above is new although after the summer of 2007 it is more relevant than ever.

This extended version of this article published with permission from John Marshall, July 2008

Note: The views put forward in this article are those of the author and may or may not be supported by the web site owner.

Author profile

John Marshall is a keen gardener and chrysanthemum exhibitor. John has been growing chrysanths for about 12 years and is the Chairman of High Wycombe Chrysanthemum Society. John is a member of the UK National Chrysanthemum Society.
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Throughout this article you will find pictures of John's blooms in his greenhouse and under his covers.
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