What we learned in 2021

In 2021 we added 323 observations to the database for 226 cultivars! The study took place in 5 of the 7 planned gardens. There was insufficient rust in the other 2 gardens to do observations.

Rust eradicated in Central Texas?

In February of 2021 Texas was hit with the most extreme cold weather in the last 30 years. We had 7 days where the temperature never got above freezing. The low at the authors house was 4 degrees. We also had a foot of ice and snow.

For the first time since daylily rust became a problem it disappeared from most gardens around Austin. Gardens that overwintered daylilies in pots and protected them or bought plants in in the spring, quickly had rust reappear.

Not so fast, the Authors garden!

Not since daylily rust hit in the early 2000's did we have a chance to see how rust spreads. In the authors garden we had 2 pots with daylilies with some rust. By June two beds which those 2 pots were nearby had a substantial amount of rust. The most susceptible plants were the first to show rust. By November any susceptible plants had substantial rust. Highly resistant plants showed either no or a tiny ammount of rust by November.

In June the author moved one of the pots to a bed isolated from the now infected beds. From this bed two larger beds are 20 feet and 100 feet away. By September susceptible plants in all three beds had substantial rust, and by November most susceptible plants had lots of rust. Like the first 2 beds highly resistant plants had little or no rust.

The author has 2 additional isolated beds where no attempt was made to infect them. By September a number of daylilies were showing rust, and by November most susceptible daylilies had substantial rust. We have no idea how rust got to these beds.

Rust in other gardens in the study

The Threatte garden added a couple daylilies that came from Florida in very early spring. By June rust was spreading and by November rust was throughout the garden.

The Renner garden was one where pots were protected during the freeze, and rust was throughout by November.

The Tanglewild garden, based upon reports from the owners did not have daylilies overwintered in pots but had very substantial rust throughout most of the beds by November. One bed which is isolated by a row of tall cannas from the other beds had very little to no rust. I believe the cannas blocked the rust spores from getting to the daylilies in that bed. No observations were taken from the bed.

Zilker Botanic Garden which houses the Austin Daylily Clubs 4 beds, had no rust through June. However by October rust had appeared in the lower bed and by November rust was observable in all the beds. Rust was not as bad as the previous gardens, probably because it got a later start. Members with rust in their gardens worked in the beds spring through fall. Zilker is quite isolated from any daylily gardens by probably miles. No daylilies were added until late November. The rust either overwintered, or more likely was brought into the garden on either shoes or clothes. The layout of the beds provides no resistance to rust moving easily throughout the garden. The less severe rust is almost certainly due to less time to spread. A limited number of cultivars were observed due to the less severe rust. Only cultivars literally next to a very rusty plant were observed.

The Ellison garden had substantial rust throughout the garden by December. It is likely that rust was introduced or returned by late spring. Whether the rust overwintered or was brought in is unclear.

The Singleton, Sanchez-Ruiz and Lochbaum gardens reported no rust. The author visited the Singleton garden in late November. The garden has approximately 200 cultivars. No daylilies were overwintered. The daylilies were amazingly pristine, like going through a time machine to prior to 2000. No rust was found on any daylily planted prior to February. The owner added a few new daylilies this fall and rust was observed on two of those. This garden makes me believe that without a overwintered source of rust most daylily gardens in Central Texas would be rust free.

In early December the author visited the Sanchez-Ruiz garden. A small amount of rust was found on a couple of plants. There was no obvious source of the rust, plants that were brought in throughout the year had no rust. I suspect the owner goes to great lengths to pull infected leaves when they appear. The garden has about 250 cultivars and all looked very pristine.

Conclusion on how fast rust spreads

Even in a garden with NO rust, a single daylily with substantial rust can infect an entire garden in a year. A highly suceptible cultivar even when separated by 100 feet or more from a daylily with rust, will likely be infected.

How do rust scores translate into how a clump actually looks?

When observing a clump we look through the clump for any leaves with rust. If no rust is visible we look at several older leaves under a magnifying glass to look for signs of rust. Daylilies with low rust scores have been thoroughly examined.

In general cultivars with scores < 1.1 will look pristine in a garden. The higher the score the more likely you will easily see rust on a clump. The more observations of a given cultivar and preferably in multiple gardens, the more likely the score accurately reflects the cultivars resistance.

A cultivar with a score of 0.00 is very likely to never have rust visible, with enough observations this is a possibly immune/highly resistant cultivar.
A cultivar with a score of 1.00 might have one to a few spots on a few leaves and is unlikely to be visible.
A cultivar with a score of 1.01-1.09 has slightly more rust but is still unlikely to be visible this is probably a highly resistant cultivar.
As scores go from 1.1-2.0 it gets increasingly likely that some rust will be visible, these are somewhat resistant cultivars.
From 2.0-2.9 you probably will see the rust these are somewhat susceptible cultivars.
From 3.0 to 5.0 you will see the rust, and the plant will possibly look disfigured, these are highly susceptible cultivars.

Diploid versus Tetraploid likelihood of being rust resistant.

The following counts in each level of rust indicate the number of tetraploid’s out of the total in that category. It has been estimated that about 1/3 of all diploid cultivars show some level of resistance. Tetraploid’s have twice the number of chromosomes as diploids 44 versus 22. The odds for any tetraploid cross acheiving the highest rust resistance possible from a given cross are substantially lower than the same diploid cross. For the following data to be truly meaningful, the sample of daylilies would need to be random which this study is not. It is skewed towards identifying highly resistant cultivars. However it is interesting to note how the percentage of tetraploid's goes up as the resistance goes down. I am not sure exactly what this means. At a minimum it probably means that highly resistant diploid's are easier to find than highly resistant tetraploid's.

0.00 9 tets of 29 or 31%
1.0-1.09 23 tets of 62 or 37%
1.1-1.99 53 tets of 76 or 70%
2.0-2.99 29 tets of 36 or 81%
3.0-5.00 21 tets of 22 95%

Possible techniques to mitigate Daylily Rust in your garden

After closely observing several hundred cultivars this year here are some techniques to mitigate rust.

Think of highly susceptible and somewhat susceptible cultivars as sources of high rust pressure in a garden.
Removing as much rust pressure as possible will give the highly/somewhat resistant cultivars a chance to outperform their score, since they were observed under high rust pressure.
Remove all susceptible cultivars from the garden. If the cultivars score is > 2.0 remove it.
Plant highly resistant cultivars between somewhat resistant cultivars. We observed many, many cases where a resistant cultivar only had rust on the side facing a more susceptible cultivar. This minimizes the rust pressure.
If you must have susceptible cultivars, isolate them as far away as possible from your more resistant cultivars. Consider adding a screening row of plants between these and the other cultivars, to help block the spread of rust.

If you want to trial daylilies for rust resistance

Ideally plant the trial plants between highly susceptible cultivars. You will quickly gain an understanding of how resistant the trial cultivar(s) are.
Be patient, wait until heavy rust is near the trial plants. You may have beds where not enough rust is nearby, don't observe plants in those beds.
Deciding when to begin taking observations is the most difficult part of trialing daylilies for rust. If you are doing just a few, wait until November or just before your first frost.

Summary of 2021 cultivars studied

A word document summary with ranked scores of the daylilies studied in 2021 can be downloaded from here.