In this fourth installment we will look at the importance of identifying recurrently resistant cultivars, the mindset required to make a good resistance breeder and the importance of exposure to pathogens in a resistance breeding program.
The Importance of Identifying Recurrently Resistant Cultivars
As well, there are many examples of cultivars that are “evergreen” and “semi-evergreen” in southern climates that behave as “semi-evergreen”, semi-dormant”, “dormant” or even “hard dormant” when grown in northern climates. While the original species may in some instances be truly evergreen or dormant in all climates, even in some of those (fulva “evergreens” come to mind as do citrina clones) their foliage performance will depend a great deal on where they are grown. Since our modern cultivars are an amalgamation of many species, we can expect the genes of foliage type to be very mixed with great variability seen throughout the many climate ranges.
While their leaves died back that winter and the rust was gone, many of the plants were so weakened by the heavy rust of the previous year due to also having low tolerance that they performed very poorly with lowered fan number, bud counts, and branching the next year. As well, a great many of their seedlings were also highly susceptible, like having produced like in those cases. Most of these were removed from any breeding consideration with a few being used in salvage projects, while others that I retained are now here merely to use to ensure heavy rust to inoculate my seedlings when rust does appear.
Both examples from north and south indicate that passive selection and removing rust magnets, rather than actively selecting for highly resistant or immune seedlings, can still be of benefit in a breeding program and that just the act of using a known resistant cultivar can increase the odds of producing rust resistance, even in situations where no selection pressures from rust infection are possible. So let us now consider the importance of identifying recurrently resistant cultivars.
It doesn’t matter how old or simple the cultivar, though many may be neither old nor simple. All that matters is that those showing long-term resistance in many locations give the breeder a jumping-off point for potentially increasing the levels of resistance in their own seedling population. A further point is that when such cultivars also have many offspring that show recurrent resistance in many locations, we can then say that such cultivars and their family lines may represent a high level of breeding value for resistance.
The more anecdotal evidence that can be gathered, the better an insight we can gain into the rust resistance and potential breeding value of a given cultivar for rust resistance and this could then also build a foundation that professional researchers could use to establish proven resistance under proper controlled conditions. As professional researchers do more research, giving us a greater understanding of both rust strains and resistance factors, the greater our ability to breed for resistance will become, but at the same time, we mustn’t undervalue the observations of the hobbyist breeders. Both the anecdotal information of hobbyists and the empirical information of researchers are important in the pursuit of knowledge.
Beyond the observation and physical work required, my experience suggests that a certain mindset is required to pursue resistance breeding in an active fashion. That mindset requires focus and dedication, but perhaps the most important attribute required is detachment and/or a low threshold of sentimentality, and a large dose of practicality seems to help as well.
Resistance breeding requires a certain detachment to these things and an ability to drop sentimentality and cull where necessary and ‘salvage projects’ should remain special cases. Not everyone can do this and one must do some soul-searching before they begin to pursue such a program. The question one needs to ask oneself is, “Can I really cull and if so, can I cull or not breed from expensive, stunning, rare or advanced individuals?” If the answer is no, then perhaps this type of breeding is best left to those who can answer yes to that question, or a more passive program is most suitable for you.
In the next post, we will begin to look at actual breeding methods, schemes and systems that can be used in the resistance breeding program...