Daylily Rust and the Black Stem Rust of Wheat: a "new" disease of Daylilies

A fungus called the Black Stem Rust of Wheat troubles wheat and many other grain crops in the grass family. Puccinia graminis of wheat is closely related to Puccinia hemerocallidis of Daylilies. Since the two are related, one might we might learn by studying the better-known disease of wheat.

The Black Stem Rust of Wheat and Barley is a potential threat to the world’s food supply. It has been estimated that over $500 billion are lost to the rusts on the cereal grain crops each year. Wheat stem rust has been recorded as a problem for over three thousand years. It has even been suggested that the fall of the Roman Empire, following a series of rainy years, may have been partly due to the lack of grain food due the Puccinia.

Puccinia graminis is heteroecious, in that it must grow on two or more different plants in order to complete its life cycle. Upon germination of the zygote, four basidiospores, resulting from meiosis, are produced and are blown by the wind and will germinate if they land on the Common Barberry, Berberis vulgaris (the resistant local barberry is the Japanese Barberry). The hyphae grow within the leaf of the barberry absorbing food and nutrients from the plant. In time structures called pycnia will form, usually on the upper surface of the leaf. The pycnia or spermagonia produce both spermatia and receptive hyphae. The spermatia will be transferred to other spermagonia by insects and there the cell fuses with the receptive hyphae cell. It is this fusion between two cells, which is necessary for sexual reproduction to occur. The nuclei do not fuse at this time, so growth will produce cells with two separate nuclei, the dikaryon condition. Dikaryon hyphae will grow throughout the barberry leaf and then produces reproductive structures on the lower side of the leaf called aecia. Each aecium will produce numerous aeciospores which will be wind blown.

Movement of infection due to windblown urediospores

When the dikaryon aeciospores land on wheat, they will germinate and form a dikaryon hyphae throughout the stem and leaves. Because the fungus is taking food from the plant for the process of growth, the wheat plant is weakened and produces a poorer crop and weak stems. The hyphae in the wheat plant will produce reproductive structures called uredia. These produce dikaryon orange reproductive cells called urediniospores or summer spores. Urediniospores are wind blown and will germinate when they land, many miles away, on wheat to form more hypha and more urediniospores. Because of the large acreage of wheat grown in the southern states today many urediniospores are wind blown and infect northern wheat. Urediniospores seldom are able to re-infect northern wheat after overwintering. There have been incidences, however, where heavy snowfall has protected the urediniospores and allowed them to re-infect the plants in the spring. While some have been known to live through the winter, they rarely do so in a position to be wind blown and re-infect wheat. In the fall the uredia will change and produce black spores instead of the orange urediniospores. The structures are then called telia and the black spores are called teliospores. Each of the two-celled teliospores will eventually contain two, diploid zygotes, which are able to withstand very cold temperatures and over winter. Over wintering of teliospores may occur on the ground near the plant or on dead wheat leaves or stem. In wheat, teliospores do not germinate on barberry in the Deep South because the spore requires extended periods of freezing weather.In the spring, the teliospores will germinate and produce basidiospores, which will infect barberry all over again.

Life Cycle of Puccinia graminis and P. hemerocallidis

In an attempt to break the cycle, there was a concerted effort beginning in 1918 and continuing, more or less, to the present, to eliminate the Common Barberry and other susceptible species from much of the United States as a way of eliminating sexual reproduction and interrupting the cycle. It was hoped that this might eliminate the fungus as a pest of wheat. Normal sexual reproduction was, therefore, prevented. However, these fungi have the ability to fuse the nuclei within the dikaryon hypha and then have the nuclei become haploid again. So, even though sexual reproduction is not occurring, genetic recombination is able to come up with new combinations of the mutations as they occur. Crop geneticists are continually working to produce varieties of wheat resistant the Puccinia. These new strains plus the newly imported strains means that individual wheat varieties usually remain resistant to the fungus for about seven to ten years by which time the fungus will have mutated to the point that the wheat variety becomes susceptible to the disease (the government has cut funds for developing new genetic varieties of wheat). Because wheat is actively growing in the southern states during the wintertime, the fungus remains active twelve months of the year. This means that urediniospores can be produced all the time. In the springtime, as wheat begins to grow farther north, urediniospores are wind blown and infect the susceptible wheat varieties farther and farther north, well into Canada. One classification of the means of measuring wheat genetic resistance to P. graminis is as follows: Immune. No uredia Very Resistant. Uredia minute, scattered, surrounded be dead tissue. Moderately Resistant. Uredia small to medium in size, isolated, surrounded by chlorotic or necrotic (dead) tissue. Moderately Susceptible. Uredia medium in size, rarely coalescent, tissue remain alive although chlorotic zones may be present. Very Susceptible. Uredia large, numerous, confluent, no necrosis.

Diagrams and pictures are of P. graminis. Note the similarity of the life cycle of P. graminis with that of P. hemerocallidis. See: Bergeron and Ono titled "An Investigation of Daylily Rust on Hemerocallis, Hosta and Patrinia in Japan", which appeared in The Daylily Journal, Vol. 59, No. 2 Summer 2004.

It is obvious that only a totally immune variety of wheat will prevent the disease from spreading to other varieties. Culture practices are little help in the control of the wheat rust. The planting of resistant varieties of wheat is the best practice, although resistant varieties do not remain resistant for long.

A few, somewhat effective, fungicides to protect the wheat include: Dichlone, Propaconizole, Sulfur, Tridemefon, Zinc/maneb mixtures, Zineb. As these fungicides are only slightly helpful in control, they are used only during large outbreaks. The production of resistant varieties of wheat, remains the only dependable control of this disease.

Something for Daylily enthusiasts to think about

When one compares the life cycle of Puccinia graminus of Wheat with that of Puccinia hemerocallidis of daylily we can see that the life cycle is the same except instead of the urediniospore and teliospore host being on wheat, it is on daylily. Also the spermagonia and aecia are on Patrinia instead of Berberis.

Bergeron and Ono (2004) report that there are no reports of the infection of Patrinia in the United States. I assume that a possible reason might be: a) too few Patrinia plants have been planted near plantings of Hemerocallis, b) too few Patrinia plants are in the north, where teliospores will germinate (work might be done to determine if P. hemerocallidis teliospores require extended periods of freezing temperatures like P. graminis), c) numbers of teliospores produced where they might germinate are too low because the numbers of daylilies are low or the numbers of untreated daylilies are low.

The northern growers should be aware that, in wheat, urediniospores may over-winter when protected from the cold by snow. If Daylily Rust behaves the same way, growers in heavy snow prone or mulched areas, who have infected plants, should not assume that all the spores would always be killed during the winter. Under these conditions attention should also be paid to possible differences between evergreen, semi-evergreen and dormant daylilies, as well as resistant varieties, under these conditions as the rust might be more likely to over-winter in living leaves.

Those daylily breeders who argue that we should only be hybridizing resistant varieties should take into consideration that if the Daylily Rust behaves as Puccinia graminis, each new resistant daylily variety will have a period of time to grow before they become susceptible to the new strains of the fungus. With the daylily rust, the time period may not be expected to be as short as ten years, as in wheat, due to the relatively small numbers of daylily plants in North America compared to wheat plants.

So long as we have areas of daylilies actively growing in the southern states all the time, there will be a constant reservoir of urediniospores available for infecting the northern plants. One advantage that northern growers might have is that the number of acres of southern daylilies is quite small in comparison to the number planted in wheat. Therefore many fewer spore are produced by daylilies, in comparison, and there will be fewer blown north.

Another advantage northern growers have is that if the plants get infected by the southern wind blown spores, it will be later in the season. If infection occurs after blooming, the flowers would not be affected but seed maturation and plant hardiness might be. It is possible that many northern growers might not be affected until very late in the season and therefore there will be little, if any, noticeable affect on the plants.

The importation of southern grown daylilies into the northern gardens during the spring might be one way to increase rust problems in the north. If the fungus is unable to over winter in the north, then bringing southern grown daylilies north during the late summer might not be a problem, assuming that they are strong enough to live over the winter. Growing potted daylilies in a greenhouse or under lights during the winter may also cause rust problems in the north. Plants imported from susceptible regions to the north should be isolated and treated as though they are infected until it has been determined that they are free of the fungus.

Time will tell if the northern daylily growers should really be concerned with the Daylily Rust. It is hoped that at least the northern growers will be able to get by even though the plants might become affected late in the season.