Terence Robinson, Lailiang Cheng, and Mario Miranda Sazo
In the last few years, we have studied nutrient levels in ‘Honeycrisp’ fruit as influenced by rootstock and found that some rootstocks impart higher potassium and nitrogen levels in the fruit than others. It does not seem that they have less Ca in the fruit, but the ratio of K/Ca or N/Ca is elevated with some rootstocks more than others. It appears that some rootstocks are more efficient at taking up K and N than others. This leads to more bitter pit with some rootstocks than others.
The issue of K fertilization is interesting because we found in the 1990’s that K was essential for large fruit size and high yield of ‘Empire’. Our work in 2008-2009 on ‘Gala’, another small-fruited variety, also showed that high K levels in leaves (1.6%) and fruit (0.8%) are needed for fruit size and yield. Thus, we promoted its annual use with both varieties and had high targets for leaf K level (1.5-1.8%). It worked well with all other varieties until ‘Honeycrisp’ came along. We found that ‘Honeycrisp’ requires less K to have large fruit size and high yield than ‘Gala’. Thus, it should need lesser amounts of annual K2O than ‘Gala’. We continue to recommend annual applications of K fertilizers to ‘Gala’. Our work also showed with ‘Gala’ that a high yield of 1500 bu/acre will remove about 100 lbs of K2O per acre with the fruit. Thus at least that amount of annual K should be applied to ‘Gala’ to sustain that high yield. With the new data we recommend much lower amounts of K fertilizers with ‘Honeycrisp’. In addition, we also have a much lower target for leaf K level of (1.0-1.3%) with ‘Honeycrisp’ than with ‘Gala’ (1.5-1.8%).
Another factor is the efficiency of K uptake by different rootstocks. Some rootstocks such as B.9 do not take up as much K as other rootstocks. M.9 is intermediate while G.41 and G.11 are very efficient in K uptake. The low vigor of B.9 and low uptake of K is good for bitter pit, but B.9 trees almost never fill the space well enough for high yields, while G.11 and G.41 have slightly more vigor to fill the space but are very efficient at taking up K and thus can have more bitter pit in some years. Recently, we found that G.214 has similar vigor to G.41 but poorer uptake of K and thus has low bitter pit risk (the best of both worlds).
We still don’t know why some rootstocks are more efficient at taking up K and Ca than others. Probably it has a lot to do with vigor and root system size. Higher vigor usually means a greater volume of soil is explored by roots and thus the plant has access to more N and K.
Lastly, for managing ‘Honeycrisp’ and ‘Gala’ we recommend using leaf analysis, and for ‘Honeycrisp’ we also suggest fruit peel sap analysis, to evaluate how much K and N to add or not add. Honeycrisp by itself seems to be less efficient than other scions in the ability to transport calcium to fruit, while it is able to be very effective with potassium and nitrogen. If leaf K and N levels of ‘Honeycrisp’ are above 1.3% and 2.2%, respectively, then we suggest a reduction in the annual K and N applications to zero for a year. For ‘Gala’, if leaf K levels are less than 1.5%, we suggest additions of 100 lbs K2O per acre per year until that level is achieved. However, for ‘Honeycrisp’ if K level is between 1.0 to 1.3%, reduce the K rate by 25~30% that was recommended for ‘Gala’ at the same yield level. In addition, fruit peel sap analysis has been very helpful in deciding how to fertilize ‘Honeycrisp’. If the peel sap K/Ca ratios are above 25 then that is also a signal to reduce K applications to zero. If peel sap levels of the K/Ca ratio are below 25 then ‘Honeycrisp’ should receive about 40 to 50 lbs K2O/acre at a yield level of 1000 bushels per acre. Because ‘Honeycrisp’ often has 15 to 30% lower yield than ‘Gala’, the annual maintenance K application rate is often only about 50 to 60% of what was recommended for ‘Gala’.
