A Daylily Rust FAQ

Last updated 10/25/07.

Mike Huben

The idea of this FAQ is to give a quick overview, to fill the gaps that the FACTUAL pages leave, to debunk the nonsense, and refer people to the factual pages. I will try to keep it up to date as new information becomes available and as people suggest new questions. I pretend to no special expertise in plant pathology, but do attempt to keep abreast of developments in daylily rust. I intend for this FAQ to be primarily fact based, but there are places where I offer my opinion. I attempt to identify those places, and will gladly rectify any omissions.

This FAQ is written primarily for AHS members and not commercial growers, though it may prove useful for all concerned.

If you haven't had a basic course in plant pathology, I highly recommend this very brief introduction to plant pathology concepts and practice: Plant Disease And Plant Disease Diagnosis (http://scarab.msu.montana.edu/Diagnostics/DiseaseDiag.htm).

The few technical terms used in this FAQ can generally be found in this Illustrated Glossary Of Plant Pathology (http://www.apsnet.org/education/IllustratedGlossary/top.htm).

This FAQ is divided into 6 sections:

About the rust fungus.

Where is the best information about daylily rust?
The first daylily Rust FAQ for AHS members was written by Dr. Jean L. Williams-Woodward in The Daylily Journal, Vol. 56, No 3 Fall 2001, pp. 282-288. Unfortunately, it's not on the web, and so this FAQ will duplicate some of its content.

Sue Bergeron's Daylily Rust Information Page (http://www.ncf.ca/~ah748/rust.html) is THE one web page to bookmark on this subject.

What is daylily rust?
Daylily rust is a fungus with a complex life cycle. In the course of that life cycle, the rust switches from parasitizing daylilies to a briefly free-living form to parasitizing another plant called Patrinia, and then back to daylilies. Along the way, it changes from uninucleate to binucleate and back again and produces five distinct types of spores.

The stage of greatest concern to us now, as daylily growers, is the stage that infects and reinfects daylilies through cloned spores called urediniospores. These can be spread by wind, splash, contact, and animal (or human) transport. When they arrive on a leaf, under the right conditions of temperature and humidity they will germinate and attempt to grow through a stomate to infect the interior tissues of the leaf. When enough growth has been achieved inside the leaf, and environmental conditions are right, the rust fungus produces pustules that burst through the surface of the leaf to release a new generation of cloned spores.

Is there more than one kind of daylily rust?
There is only one species of daylily rust listed in the scientific literature. And so far (2/2002), in the United States, there seems to be only one asexually reproducing clone of this rust.

What else can this rust infect?
Here's the good news: little or nothing. Some old scientific literature reports that this rust can infect Hosta, but experimental attempts to infect Hosta with our US clone have failed, and it is quite possible that rusts on Hosta are a different species or variety. This rust is quite likely to be able to infect Patrinia, the alternate host of record, a seldom-grown but widely available genus of Asian perennials that is not naturalized in the United States. But tests to demonstrate experimental infection of Patrinia have not yet been completed (11/2001). In addition, unpublished Japanese experimental attempts to infect Hosta with aeciospores from Patrinia (that did infect Hemerocallis) have failed. Daylily rust on Hosta is so far unconfirmed. See the Hosts Of Daylily Rust web page for the details. (4/2002)

Where and how has this rust spread?
Daylily rust originated in Asia. It has now been reported from most parts of North America. It is present in Central America, and has recently been introduced to Australia.

This rust has spread long distances between gardens and growers primarily by shipping of infected plants. There are a few reports of infections that might have arisen otherwise, but the vast majority of infections are directly attributable to receipt of a plant that seems to have borne the infection. It seems very unlikely that these infections have been spread long distances by wind: that only seems to happen with crops that cover large amounts of land.

A report of frequent spread by wind (presumably) over indeterminate distances (probably up to a few miles) to at least 30 non-AHS gardens in Florida has come in from Dave Talbott. Infected plantings create "spore pressure": the presence of spores in the atmosphere. Spore pressure declines with the square of the distance, and the lower the spore pressure, the less the probability of infection. As more reservoirs of infection develop in overwintering areas, the spore pressure will increase, and we'll see more local transmission by wind. Long distance transmission by wind of daylily rust has not yet been shown to be likely.

Within a garden or field, rust is HIGHLY CONTAGIOUS. Wind, splash, contact, and animal transport can spread rust spores throughout a garden, infecting it thoroughly in a very brief period.

Daylily rust is not thought to be systemic. In individual fans, rust seems to pass from leaf to leaf by infection by spores alone. Not by growing though the tissues of the crown. It is not clear yet how far infections in leaves grow.

How bad is daylily rust?

What does daylily rust look like?
Daylily rust produces a disease whose most characteristic sign is raised orange spots that leave a powdery streak when wiped with a tissue. A sign is a visible evidence of the disease causing organism itself, and the orange spots in this case are masses of fungal spores. Rust also has symptoms, such as yellowing of portions of leaves. Symptoms are visible reactions of the host to the disease causing organism.

These symptoms and signs are well illustrated at the Daylily Rust Information Page.

How do I search for daylily rust?
When searching for daylily rust, it is most important to check the undersurfaces of older leaves according to Pat Stamile. Other sources also suggest especially checking the tips and bases of the leaves. If you have an opportunity to visit a garden with daylilies rust, that would be a good way to learn to spot it. Confirmation that you have found rust is available: see Who to Contact if you Find Daylily Rust.

What effects does rust have on daylilies?
Like most diseases, rust contributes to the general debilitation of the plant; both directly by consuming plant nutrients that could be used for plant growth and indirectly by damaging the plant causing less photosynthesis and increased water loss. Rust does not apparently result in immediate death. However, under some conditions this might cause decline of the plants or predispose them to death from other causes. A report from China does indicate severe setback to production from the plant (10 to 50%, presumably weight of buds).

An additional factor is that rust is conspicuously ugly on many daylilies, perhaps much worse than leaf streak.

Why should I avoid daylily rust?
The primary reason is because rust will disfigure many of the daylilies in your garden unless you control it. When you have rust, other people who don't have it yet will want to avoid your plants. Rusty plants can require much more grooming to remain presentable.

A secondary reason is that controlling rust requires expensive and time consuming application of fungicides, comparable to the labors of Rosarians (rose fanciers). There are reports that controlling for rust adds 20% to the costs of commercially growing a daylily. And while the fungicides involved are more benign than many other pesticides, they still present a risk that it would be better to forgo.

The question boils down to whether you want to avoid unnecessary diseases enough to attempt to avoid rust. Most people who have rust wish they never had it.

Do all daylilies catch rust?
Apparently, yes. Matthew Kaskell reports that all daylilies in his garden show rust, but in differing amounts. Others report that species catch rust too. A daylily that could not catch rust would be properly termed a non-host or insusceptible. Immunity in a scientific sense means insusceptibility due to training of an animal immune system (plants don't have immune systems), but since no daylilies seem insusceptible, the term probably shouldn't be used as a casually either.

All daylilies seem to be susceptible to rust, as opposed to resistant to rust. Resistance means relative difficulty for infection to occur. No resistance has been reliably reported yet in daylilies.

The variation that Matthew and others report is properly described as tolerant versus sensitive. A tolerant plant will have the disease, but not be much affected by it. A sensitive plant will be severely affected by the same disease.

Which daylilies are most rust tolerant?
It is just too early to say. This topic is already being hotly debated, as we would expect differing growing conditions to affect the progression of the disease differently. Development of pustules and spores is apparently dependent on temperature and humidity, for example. Health of the plant might be another issue: Florida observations of "susceptibility" of PARDON ME, a dormant, were reportedly made after a winter of no dormancy, when dormant plants grow poorly. Other reports from northern gardens with rust claim that PARDON ME is relatively tolerant of rust (according to Darrel Apps.)

Rebecca Board's Rust Survey is attempting to survey tolerance across many gardens. This should help a great deal as the data starts to flow in.

The good news for breeders, according to Matthew Kaskel, is that it seems to be very easy to screen large numbers of seedlings for tolerance at a very early stage. This doesn't imply that they will exhibit tolerance under all conditions, but it does allow one to screen out seedlings that are highly sensitive under the grower's conditions.

In the north, tolerance my not be as important as dormancy. If dormant daylilies can shed the infection with their leaves, gardens would be able to start the year free of infection. In areas of sufficiently severe winter, it's possible that even evergreens might be able to shed the infection. We'll learn more about this possibility in 2002.

If you have rust in your garden.

Where can I buy from?
Anywhere. It doesn't matter with respect to rust: you will not get any new rust variety. (Yet.) You can control rust on the new plants the same way you control it in your garden.

What is the difference between control and eradication?
Eradication is control by killing of all of the disease causing organism. Less extreme forms of control have the purpose of bringing the damage down to tolerable levels.

Should I attempt to control, eradicate, or tolerate daylily rust in my garden?
This is a value judgement. If you are selling daylilies, you'd be best off with eradication, though no control program has yet been convincingly shown to result in eradication in large gardens yet. At the opposite end, if you wish to breed tolerant daylilies, you must NOT control rust, because you cannot identify tolerant seedlings without testing them in the presence of rust. For ordinary gardeners, the question is the tradeoff between bad looking foliage and the management of a costly spray program.

How can I tolerate daylily rust?
Early reports (2/2002) from the southern US say that growers are simply eliminating varieties that show severe rust infections and retaining varieties that show good tolerance. This sounds quite practical for the south, where due to rapid increase, clump replacement takes little time. It might be a much slower process in the north. Northerners might want to plan and start the elimination and replacement (once the susceptible varieties are known) before they get rust. It would be very helpful if southern gardeners announced which varieties they were eliminating as showing severe rust.

How can I control or eradicate daylily rust?
This is a subject beyond the scope of this FAQ where prescriptions are rapidly evolving.

I'll be happy to link here to recent recommendations for spray programs when I see them on the web.

But a brief, general outline was presented by Dr. Williams-Woodward at the Sept. 2001 Region 15 fall meeting:

"Cutting back infected leaves to the ground and removing from the garden is recommended to reduce the number of spores present, remove most of the infected but not yet fully mature infections, and remove most of the green surface needed for germination. Plants will need to be sprayed as they grow out to finish the last remaining infections and prevent the remaining spores from germinating."

"Rust should be unable to survive a cold winter if the plants are _completely_ dormant, showing no green anywhere, and if there is no alternate host present. This is based on the experience of only one winter (a cold one) in the speaker's own garden with only dormant varieties."

"It was recommended to cut and remove all foliage from the garden in the fall, and possibly to continue to cut back evergreens during the winter months to reduce the odds of rust surviving the winter. The more foliage is removed on new arrivals, the less chance of rust, though obviously one cannot reduce the odds to 0 without killing the plant."

[One further note of explanation here: when removing possibly infected foliage, care should be taken to avoid later airborn distribution of spores from the cut foliage. It could be bagged, burned, or buried, for example.]

Pat Stamile posted an excellent explanation of the recommendations for northern gardens to the email robin, available here.

Dave Talbott has passed along an outline of his current (2002) spray program for his commercial garden. It is here for illustration only, not recommendation. Home gardeners may not need such a thorough program, as they may prefer to merely control rust.
We are back on a weekly spray program now that the weather is moderating. High 20's,low 30's and upper 40's are not conducive to pustule production and we only spray bi-weekly during the winter months. This is my schedule which has kept me asymptomatic for 2001 and into 2002. On a weekly basis I spray alternately with:
Chlorothalonil (Daconil Ultrex)
Triadimefon (Strike 50 or Bayleton)
Mancozeb (Dithane or Fore).
I spray plants to run-off and use a spreader sticker. Although it may be overkill for the homeowner but several times a season I will substitute Azoxystrobin (Heritage) for Triadimefon.There is a cheaper form of this material named Abound. However it has only 20% of the active ingredient rather than the 50% of Heritage. We dip each plant in mancozeb and let it dry before shipment.We know the precise dosage to use with this material to kill spores. We do not know with clorox.

Are there natural controls that can be used to keep rust manageable?
Not really. I am not aware of any predators, parasites, or diseases of rust fungi that are exploited for control of any pest species of rust. Quarantine, sanitation, fungicides, and tolerant varieties are the major methods of control.

Bothrotes canaliculatus, a Tenebrionid beetle that feeds on fungi, has recently been reported feeding on rust spores, but they are unlikely to eat a significant amount of rust.

Should I sell plants from my garden?
"It is difficult to get a man to understand something when his salary depends on his not understanding it." Upton Sinclair

This is a hotly debated subject. The current wide distribution of rust within AHS gardens is primarily due to popular central Florida hybridizers who did not shut down their sales, in my opinion. AHS members were poorly informed of the threat and its significance; the Florida hybridizers had serious conflicts of interest in publicizing this menace, and the AHS did not publish any information to the members until long after the shipping season.

If you have had rust in your garden, there is a significant chance that you might sell some plants with rust. Even if you have clean looking plants, even if you have been spraying diligently, the plants might well have concealed infections that will be revealed in as long as three to five months after leaving your garden. There have been numerous reports of this happening with plants from the Florida hybridizers in 2001. It is possible that the latest spray regimes and sanitary practices at these growers have freed their plants of rust, but unfortunately there is no easy way to test this in advance. No major grower that I know of is even attempting to test this adequately. In 2002, this problem may be continent-wide, depending on how well rust overwinters.

If you are selling to gardens that have rust control practices in operation, either quarantine or spraying, this should not be a problem.

In short, the major ethical dilemma is whether the customer understands and is willing to accept the risk of purchasing from your garden. Seller's responses to this dilemma have ranged from shutting down sales entirely (at least two popular Florida hybridizers did this) to stonewalling and downplaying the issue while continuing to sell freely. My opinion is that at the very least, customers should be informed of your garden's rust situation and control practices.

How will daylily rust affect my breeding?
Rust infection can be considered an opportunity for addressing an important new breeding goal. Matthew Kaskel posted an excellent plan for breeding for rust tolerance. The key point is that it is simple to select for rust tolerance out of large populations of young seedlings by simply infecting them all and keeping the best looking. Breeders will not be able to select reliably for rust tolerance unless they expose their plants to rust.

Tolerant plants thus produced might act as Typhoid Mary's, carrying hidden infections with them. However, here as in other crops, tissue culture can rescue these varieties from infection far more certainly than current fungicides. The cultured plantlets could then be grown to maturity in clean, isolated fields. Similar techniques are used for many commercial crops.

It may be necessary to outcross to rust tolerant plants from other breeding programs, if yours doesn't happen to have enough tolerant varieties.

Alternatively, seed produced from rusty plants can be surface sterilized with dilute bleach or hydrogen peroxide, and germinated free from rust. Then the seedlings can be grown in a garden free of rust. Pollen from rusty plants should also be safe, especially if cut buds are surface sterilized before they open.

Another extended discussion of breeding for tolerance is presented by Dr. Raoul A Robinson at his Return To Resistance web site. A short (but technical) summary of his methodology is: The Acceptance of Horizontal Resistance in Crops.

If you do not have rust in your garden.

How can I reduce the risk of getting daylily rust in my garden?

None of these guarantees you'll avoid rust, but they all improve your chances of avoidance.

Where can I buy from without bringing in daylily rust?
There's no certainty here. Rust is present in most states now, and has appeared in the gardens of many hybridizers and other sellers. You can ask them questions like these: Has rust appeared in your garden? In nearby gardens? Have you been spraying to prevent or control rust? Have you sent out plants the past season which were reported to have brought rust to new gardens? My personal opinion is that sellers ought to place this sort of information in their catalogs; some are starting to.

There is a conflict of interest between sellers who have had rust and buyers who might wish to avoid rust. If a seller provides information about the rust situation, it might deter a sale, and few small businessmen want to risk their business that way. Several noted daylily growers have told me this explicitly. None of them want to send a rusty plant, but none of them want to be shut down by agriculture departments, lose business, or be sued because they implied their plants were rust free. Thus relatively few mention rust in their catalogs. All that I've asked have responded to direct inquiries.

Despite vigorous spray programs, many central Florida sellers who had rust in their gardens sent out rusty plants in 2001. Some of the plants showed no signs of rust until as much as 3 to 5 months later, and then developed obvious infections almost overnight. There is no way yet to be sure that plants are rust free while spraying them to control rust, though they might be less likely to be infected if they are from a garden which never had an outbreak. There is no certain security.

In 2002, you might be able to purchase rusty plants from almost any state, because the disease has been introduced nationwide to many sellers.

Nor are mass market sources or ordinary nurseries safe. There were many reports in 2001 of rusty plants available for sale at WalMart and other sources.

What is a good quarantine procedure for incoming plants?
One method of quarantine (used by Darrel Apps) is to peel leaves off single fans until only a nubbin is left (or you could cut close to the crown, below the soil line), and then soak in a fungicide such as Heritage. During regrowth in a greenhouse (or indoors), plants are carefully inspected: any signs of spots on foliage cause the process to be repeated. The period of quarantine apparently needs to be a full growing season or more: Darrel plans on two years.

Pat Stamile has contributed this advice:
If there are any plants you are unsure of I would break off the outer two or three leaves right down to the crown. Leave no bits of these outer leaves. Cut the remaining foliage down to 1-2 inches and dip the plant in Daconil. If Daconil is not available use a 10% bleach solution or a solution of 2 oz. of Zerotol in one gallon of water. The scapes and plants will be smaller than normal but you should be able to avoid rust. Do not just cut the plants back and dip without removing the outer leaves. Dr. WW reported that she saw spores and pustules on these outer leaves which were only one half inch above the crown. Remember no treatment I know of can remove rust once it is inside of the plant which is why you especially need to remove these leaves and cut back the plants.

This procedure is pictured by Susan Bergeron here.

How can my club run an auction or sale in this era of rust?
Clubs ought to be sensitive to the problems of rust, and not spread rust casually. Here is one straightforward set of ideas for how to manage sales conscientiously.

This year, the New England Daylily Society is limiting its spring sale and auction to "companion plants" (horrible term!). The membership has concluded that we'd like to be careful, and not sell our own plants until we know more about the rust status of the source gardens, and know more about how well rust overwinters here. We expect to then reconsider for our normal August auction.

Can we use information about rust seasonality to avoid spreading infection?
In 2001 (according to an informal survey by Mike Huben), plants coming from central Florida in April or earlier tended not to carry rust. Plants from May or later frequently carried rust, sometimes developing three to five months after the plant was received. If this observation can be repeated, early shipment might be a way of reducing transmission of rust.

If rust infections in the north prove to be tender, and easily eradicated by winter and dormancy, then a strategy of buying after the northern bloom season would minimize rust problems.

Erroneous information.

Rust is inevitable.
Death is inevitable, but there's no reason to want it soon. Even if we assume that rust is here to stay (at least in southern regions -- it might be eradicated in northern climes), there are advantages to deferring the introduction of rust.

This rust fungus is very easy to eliminate.
Thus spake a famous hybridizer in late February, 2001. Subsequently, numerous gardeners reported rust first developing on plants received from him. It is very easy to mistake control for elimination (eradication). Unfortunately, nobody is seriously testing for eradication from their gardens except by sending out plants to uninfected gardens. In 2002, we'll find out the hard way if current attempts at eradication have worked. It is hard not to be skeptical of claims of eradication, since they are based on guesswork at this stage.

If you grow plants organically or otherwise well, they will resist rust better.
Actually, rusts (unlike some other diseases) attack the healthiest plants more readily.

Dipping in ZeroTol helps control rust.
This is entirely unproven. At most, ZeroTol (and other surface disinfectents such as bleach solutions and hydrogen peroxide) will kill rust spores on surfaces that are wetted by it. Spores in areas the dip can't reach (such as between closely touching bases of leaves) and existing infections inside the leaf will be unaffected. These disinfectents have no residual effect, nor any systemic effect. However, they will not likely do any harm.

What about soaps and home-brew remedies, and other "alternative" treatments?
There are a number of home remedies being tried by gardeners for daylily rust but it is too early to say whether any might be useful. None have been confirmed effective by controlled studies. Caution should be excercised when trying unstudied treatments in order to avoid possible harm to soil, plants, or other organisms.

The Brooklyn Botanical Garden booklet on Natural Disease Control has a good summary in the chapter Least-Toxic Controls of Plant Diseases.

Rust may be transmitted by insects, mites, slugs, or water supplies.
These are exceedingly remote possibilities, whose hypothetical importance is dwarfed by the well-known transmission mechanisms of shipping infected plants between gardens and clouds of airborne spores within gardens. They are not considered for most other, commercially important rusts either.

Phytosanitary certificates guarantee a rust free plant.
Phytosanitary certificates are a statement that an inspector did not observe pests and diseases that he was looking for. Inspectors are not omniscient and infallible: they're underpaid civil servants who have trained to spot particular problems. Phytosanitary certificates improve your chances of avoiding noxious pests and diseases, but are no promise.

Rust definitely won't tolerate a frost.
Nobody has yet determined the limits of overwintering of this daylily rust strain in the USA. There is daylily rust in Siberia, but it is not known how it survives severe winters there. Whether this is the same strain, whether there is a different alternate host, or whatever, overwintering outside the deep south is an open question.

Yet to be known.

You can't fill factual gaps with facts: you have to fill them with statements of what is not yet known. That way people know that they don't need to search for the information, and they also know where they can contribute new information.

Can rust be eradicated from a large garden?
This has yet to be demonstrated.

What is the phenology of rust?
Phenology is the study of the timing of natural cycles and how they relate to each other. In this case, we're interested relating the developmental stages of rust (such as the emergence of pustules) with behaviors of plants such as bloom times or leaf emergence.

An excellent example and explanation of phenology is at the Daylily Spring Sickness Phenology Page.

To the best of my knowledge, phenological information on this rust has been essentially undocumented in the USA, and never assembled. Because of the huge range of climates, phenology might require very different indicators in different regions. This would be a nice project for somebody, and the results could be placed in this FAQ.

What is a good test for whether a daylily has hidden rust?
There is no quick test. A fairly safe, slow test would be to bag the plant in a plastic bag in the shade for several months, maintaining the temperature, moisture, and high humidity needed for spore production. The optimal conditions (heat, humidity, etc.) for revealing or concealing infection are not yet known. However, apparently fungicide applications and/or summer heat can conceal infections for months. This is possibly how rust recently evaded Australian quarantine.

How debilitating is rust, and under what conditions?
This hasn't been measured yet.

What could be worse?
New strains of rust could make control much more difficult. New strains might be fungicide resistant, more damaging to tolerant plants, or might overwinter better in the north.

What parts of the plant actually bear infection?
While we know that the leaves bear infection, we don't yet know how far down into the leaf base the infection can extend. Scapes too can be strongly affected. I'm waiting for US reports on whether buds or flowers display infection.

How well can the rust overwinter in the north?
There are many questions that fit this category. Is the alternate host necessary for northern overwintering? Can the urediniospores produced on the leaves remain infectious through the winter? Will mycelium survive inside parts of leaves which remain alive over the winter ? Do the different growth habits of daylilies affect overwintering of rust in the north? (Here is where a more accurate classification than the traditional dormant/sev/evergreen might prove important. Pat Henley has proposed two factors: continuous vs discontinuous growth, and deciduous vs nondeciduous. Two other factors that might be important are freeze tolerance and ability to grow at low temperatures.)

A note of caution here: there are already reports in 2002 of northern growers overwintering rust on plants grown in greenhouses. It seems likely that the infection could escape to plants grown outdoors.

Are there new, systemic fungicides on the horizon?
Francois Verhaert has provided much information about systemic rust fungicides approved for use in Europe:
BOSCOR has been (successfully) used by England's Robert Grant-Downton but as far as I know the company who made BOSCOR has stopped producing it. Until recently "knowledgeable" people over here recommended the use of Bayfidan SPECIAL (Bayfidan = for agricultural use - Bayfidan Special has been tested for use on ornamentals)... BASF Europe has a new fungicide ready which will be marketed soon. It is called OPERA and is a mixture of BAS 500 (touted at MWS) & OPUS (a TRUE systemic fungicide, travelling throughout the ENTIRE plant). BUT - as always - OPERA has only been tested on large commercial crops (wheat etc.) and my contact could/would not advise me on how to use OPERA on ornamentals... OPERA has officially been granted a permission to sell in Germany.

These are not approved for any use in the USA or Canada, and that situation is not likely to change because of daylilies. It should be possible for these to be tested by US researchers.

Francois also writes about non-systemics:
At MWS, BAS 500 was said to be the most effective fungicide, better than the often heralded HERITAGE. It was also announced that BASF (the makers of BAS 500) were almost ready to introduce INSIGNIA, a spray based on BAS 500, tested on ornamentals and to be applied with intervals of about 1 month.

Will daylily rust develop new strains that will overcome tolerance or be resistant to fungicides?
In the long run, yes, but how long that will take is an open question. As a rule of thumb, all methods of combatting a disease should be explored, so that if rust circumvents one control, the others are still available. Disease control is an arms race, and we cannot simply rest on our laurels, er, daylilies. The important thing is that we must work to stay ahead of the rust. This has been successful in many, many cultivated species.

Is there a corelation between blue-green leaves and rust tolerance?
Matt Kaskell claims this is usually the case in his garden. It remains to be confirmed by others.

Is there a corelation between leaf streak susceptibility and rust?
This, too is unknown. Several people have noted that fungicide treatments for rust reduce leaf streak as well.

Will rust overwinter better in evergreens than dormants?
We'll find out soon.

What is a good way to quantify rust damage/tolerance?
Some fungicide trials have measured effectiveness in terms of "lesions per centimeter", on leaves of comparable width of PARDON ME.

Rebecca Board's Rust Survey has adopted the following scale:
Poor: The plants are covered in rust pustules and are highly disfigured and noticeable even at some distance.
Fair: The plants are covered in rust pustules and are somewhat disfigured when viewed at short range.
Good: The plants have many pustules, but their presence is only offensive on close examination.
Excellent: The plants have very few pustules, and/or their presence is only noticeable on very careful examination.

Matthew Kaskell has provided a more extensive description of how the most tolerant plants differ from more susceptible plants:
These are plants in which only the outermost leaves are infested with pustules, and there are very few pustules actually present. The pustules are mostly confined to the very ends of the foliage and along the margins of these oldest leaves. Also, there is not much tissue damage to the affected leaf and there is very little "yellowing". The pustules are on what otherwise appears to be a fairly normal and apparently healthy leaf. But the most important COSMETIC attribute that I have found to be shared by the most attractive clumps is that they all have arching, healthy-looking, unbroken foliage with the tips of each leave pointing "down". And these leaf tips are usually buried within the foliage mass, with their rust damage discreetly concealed from public view. The foliage, except at the very tips, is clean and free of pustules. Except under the closest inspection, these clumps APPEAR free of rust damage.

The Kaskell observations may be only one of many forms of rust tolerance: we should be on the lookout for others. For example, if a period of dormancy can eliminate rust, that is a mechanism of tolerance.

Judging rust tolerance across more than one location might be much more difficult: environment and plant behavior my vary significantly, and cause different susceptibility to rust.

In short, there is no good scale yet. Whatever scale is developed should be calibrated in each garden against some common varieties: perhaps widely grown Stout medal winners such as STELLA DE ORO and STRAWBERRY CANDY. The calibration will help to compensate for differences between gardens and measurers.


A work like this FAQ is not a single-person undertaking, no matter how much impetous a lead author supplies.

I'm deeply in debt to Dr. Williams-Woodward and Susan Bergeron for much information, fact checking, and miscellaneous advice. Errors, omissions, and ungraceful statements are doubtless my fault. I'm also grateful for contributions from (in no particular order) Matthew Kaskell, Dave Talbott, Pat Stamile, Francois Verhaert, Darrel Apps, Rebecca Board, Mike Derrow, and perhaps others whom I've clumsily forgotten. Remind me, and they'll be included.

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Copyright 2007 by Mike Huben ( mhuben@world.std.com ).
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