Crimson Topaz is a scab-resistant cultivar from Europe that has been gaining in popularity because of its attractive appearance and good eating quality. However, we are learning that this cultivar is rather susceptible to several diseases that cause cankers on the trunks. In Europe, trees in some locations have proven susceptible to European apple tree canker, a fungal disease caused by Nectria galligena. However, European apple tree canker is relatively uncommon in eastern United States because it prefers maritime climates that allow infections to occur at leaf scars during long wet periods in late autumn and early winter. Note that Nectria galligena is a different species than Nectria cinnabarina, a fungus that is common in northeastern United States and that causes twig die-backs and cankers at pruning cuts.
Of more concern than Nectria are indications that Crimson Topaz may be quite susceptible to trunk cankers caused by Phytophthora species. Phytophthora usually causes root rot or crown rot on apple trees, but these diseases have become less common as the apple industry adopted rootstocks that are relatively resistant to Phytophthora. However, Phytophthora can also invade tree trunks directly if cultivars are highly susceptible and if spores from the soil are splashed onto the trunks. Historically, this was a severe problem with the Grimes Golden cultivar (see this 1939 report by R.C. Baines). Now, similar problems are being reported in a few plantings of Crimson Topaz.
The first report of Phytophthora problems on Crimson Topaz was relayed to me by Dr. Mike Ellis from Ohio State, but similar problems have since been reported for other locations across the Northeast. Phytophthora is probably endemic in most orchard soils, so we don’t know why most plantings of Crimson Topaz remain healthy while others experience catastrophic tree losses (up to 50% or more?) within four to six years after planting.
Several strategies can be suggested for minimizing the risks of Phytophthora infection, but I don’t think that any of these strategies have been vetted in actual field trials with Crimson Topaz. Nevertheless, reports suggest that affected blocks can decline very quickly, so it may be wise to apply preventive measures to existing plantings rather than opting to delay control measures until the first trees show symptoms. Four strategies for dealing with this disease threat are listed below:
1. The easiest and presumably the most effective strategy is to protect Crimson Topaz trees from infection by treating them with phosphite fungicides twice per year. Phosphite fungicides are very effective against many Phytophthora species. Following foliar applications, these fungicides are rapidly translocated throughout trees. Residues within trees dissipate very slowly and therefore provide extended periods of control from a single spray. Optimal timing has not been determined, but I would suggest treating Crimson Topaz with a phosphite fungicide sometime between bloom and first cover (after trees have enough leaves to ensure good uptake) and again in late summer.
2. One consultant in Europe reports that growers there are resolving the problem by top-working Crimson Topaz on trunks of some other cultivar so as to put more distance between the soil and the susceptible Crimson Topaz portion of the tree. This solution is only useful for those who are already thinking about using top-working to change cultivars. Growers who top-work trees should always check with nurseries to determine the procedures for working with patent-protected cultivars.
3. Organic growers cannot use phosphite fungicides, but they may be able minimize the risks of Phytophthora infection by surrounding trees with a wood chip mulch so as to prevent soil (and the Phytophthora spores contained in the soil) from splashing onto the tree trunks.
4. Planting trees on slight berms will not eliminate the risk, but it could help to ensure that water will not puddle around tree trunks even during heavy rains. Water pooling beneath trees will almost certainly increase the risks that Phytophthora spores will be splashed upward onto the trunks.
Finally, the fact that Crimson Topaz is apparently susceptible to Phytophthora trunk cankers does not mean that this cultivar should be avoided. Rather, this disease alert is intended to ensure that growers who like this cultivar can continue to grow it without risking tree loss from a heretofore uncommon trunk canker pathogen.