Honeycrisp leaf chlorosis: causes and mitigation

Lailiang Cheng1 and Terence L. Robinson2

1Department of Horticulture, CornellUniversity, Ithaca, NY14853; Email:

2Department of Horticultural Sciences, NYS Agricultural Experiment Station, Geneva, NY14456

‘Honeycrisp’ is a new apple cultivar that has gained tremendous popularity over the last several years due to its unique fruit quality. A persistent problem in ‘Honeycrisp’ is a leaf disorder that develops in late June or early July when shoot growth slows down or stops. The initial chlorosis appears more towards the edges of a leaf confined by secondary or tertiary veins, and then gradually spreads to other parts of the leaf (see the photo). The chlorotic area becomes thicker, leathery, and brittle, and turns brown later in the season. The symptoms occur on almost every tree, but trees bearing a light crop have a larger number of leaves developing symptoms compared with those with a heavy crop (Robinson and Watkins, 2003; Schupp, 2003).

The initial symptoms of this disorder are similar to the damage that potato leafhoppers cause on apple leaves (Rosenberger et al., 2001). However, trees that are protected from potato leafhoppers still develop the disorder (Schupp et al., 2001). The symptoms of this disorder resemble those observed when carbohydrates accumulate in leaves of ‘Golden Delicious’ apple trees after deblossoming (Schupp et al., 1992) and in citrus leaves after branch girdling and fruit removal (Schaffer et al., 1986). The concurrence of initial chlorosis symptoms with cessation of shoot growth and the inverse relationship between cropload and severity of the disorder have led us to hypothesize that phloem loading or transport of carbohydrates is partially blocked in the leaf minor veins, which leads to feedback repression of the key enzymes in photosynthesis via accumulation of carbohydrates in the leaves.

We compared chlorotic leaves with normal leaves in terms of photosynthetic rate, carbohydrate metabolism and the key enzymes (proteins) involvedin photosynthesis (Chen and Cheng, 2004). We found that chlorotic leaves accumulated higher levels of carbohydrates, particularly starch, sorbitol, sucrose, and fructose at both dusk and predawn, but there was no difference in total carbohydrates between predawn and dusk (Figure 1). This indicates that carbohydrate export is inhibited in the chlorotic leaves. Photosynthesis was much lower in the chlorotic leaves than in normal leaves (Figure 2). Activities of many key enzymes in photosynthesis and carbohydrate metabolism (including ribulose 1,5-bisphosphate carboxylase/oxygenase and other enzymes) were significantly lower in chlorotic leaves than in normal leaves (Chen and Cheng, 2004). These findings support the hypothesis that phloem loading and/or transport is partially blocked in chlorotic leaves, and that excessive accumulation of carbohydrates may have caused feedback suppression of photosynthesis via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes.

Thus it appears that the leaf chlorosis so common in ‘Honeycrisp’ is the result of the accumulation of excessive levels of carbohydrates in the leaf. Although the exact mechanism by which excessive accumulation of carbohydrates triggersa reduction in chlorophyll concentration remains unclear, the lower chlorophyll concentration helps to decrease light absorption when photosynthetic capacity is limited by accumulation of carbohydrates. We have also found that the chlorotic leaves still absorbed more light energy than they were able to use in photosynthesis at noon on a clear day and consequently other photoprotective mechanisms were enhanced to deal with the excess absorbed light (Chen and Cheng, 2004).

Until genetic/molecular manipulations are made to eliminate the problem based on a thorough understanding of the molecular mechanism of zonal chlorosis, the best overall strategyto mitigate this problem is to sustain the sink strength of the tree to facilitate carbohydrate transport from source leaves to sink tissues (fruit, shoot tips, and roots, etc) by maintaining a large concentration gradient between the two. Cropload plays a key role in determining the overall sink strength of the tree. Leaving enough fruit on the tree is required to reduce zonal chlorosis and produce medium size ‘Honeycrisp’ fruit. However, even a moderatecropload (5-7 fruit/cm2 TCA) can result in biennial bearing for ‘Honeycrisp’, and zonal chlorosis is most severe on off-year trees. Therefore, the challenge is how to achieve a moderate level of cropload (5 to 6 fruit/cm2 TCA) each year and still get good return bloom the following year.

Figure 1 Figure 2

Figure1. Concentrations of sorbitol (A), glucose (B), fructose (C), sucrose (D), starch (E), and total non-structural carbohydrates (E) at dusk and predawn in chlorotic leaves and normal leaves. Specific leaf dry mass for normal leaves and chlorotic leaves at dusk were 106.96 ± 1.83 and 135.79 ± 2.89 g m-2, respectively. Black and white bars represent dusk and predawn, respectively. Each bar is mean  standard error (n=6). Significant difference was tested between predawn and dusk for a given leaf type and between chlorotic leaves and normal leaves taken at the same time. Different capital or small letters above the bars indicate significant difference at P<0.01 or P<0.05, respectively.

Figure2. Photosynthesis (A), stomatal conductance (B), and chlorophyll concentration (C) in chlorotic leaves and normal leaves. Each bar is mean  standard error (n=6). Different capital letters above the bars indicate significant difference at P<0.01.

Our recent trials have shown that hand thinning to this level at the normal timing (late June and early July) is too late to effectively result in good return bloom the following year. Chemical thinning trials from 2004-2008 have shown that very early aggressive chemical thinning (full bloom or petal fall) is essential to achieving good return bloom with Honeycrisp.

In addition to chemical thinning, several studies of summer applications of growth regulators have shown a positive influence of summer NAA or Ethrel sprays on return bloom of Honeycrisp. Our trials have shown that 4 weekly sprays starting in late June are better than 2 or 1 spray. We have also shown that 4 sprays of Ethrel led to increase pre-harvest drop and advanced maturity. However, despite many growers utilizing summer NAA sprays in 2005 there was widespread lack of bloom across NY state in 2006 followed by snowball bloom in 2007. This indicates that summer NAA sprays are only part of the answer and must be combined with aggressive bloom or petal fall thinning to achieve annual bearing.

It appears that to manage biennial bearing in Honeycrisp requires a multi-spray thinning program beginning either at bloom with 2 gal ATS/100 gal or at petal fall with an application of 4oz NAA+1pt Sevin XLR followed by a spray of 3oz NAA/100 gallons + 1pt Sevin XLR/100 gallons at 12 mm fruit size if needed. This should be followed by a summer NAA program of 4 weekly sprays of 3oz NAA/100 gallons beginning on June 21.

Acknowledgment

The authors would like to thank NY apple Research and Development Program to support this work.

References

Chen, L. S. and L. Cheng. 2004. CO2 assimilation, carbohydrate metabolism, xanthophyll cycle and the antioxidant system of ‘Honeycrisp’ apple leaves with zonal chlorosis. Journal of American Society for Horticultural Science129: 729-737.

Robinson, T.L. and C.B. Watkins. 2003. Cropload of ‘Honeycrisp’ affects not only fruit size but many quality attributes. New York Fruit Quarterly 11(3):7-10.

Rosenberger, D.A, J.R. Schupp, C.B. Watkins, K. Iungerman, S. Hoying, D. Straub, and L. Cheng. 2001. ‘Honeycrisp’: A darling new apple variety or just another problem child? New York Fruit Quarterly 9(3):9-13.

Schaffer, A.A., K.-C. Liu, E.E. Goldschmidt, C.D. Boyer, and R. Goren. 1986. Citrus leaf chlorosis induced by sink removal: starch, nitrogen, and chloroplast ultrastructure. Journal of Plant Physiology 124:111-121.

Schupp, J.R. 2003. Effects of chemical thinners on fruit set, yield, fruit size, and fruit quality of ‘Honeycrisp’ apple. New York Fruit Quarterly 11(3):3-5.

Schupp, J.R., D.C. Ferree, and I.J. Warrington. 1992. Interactions of root pruning and deblossoming on growth, development and yield of ‘Golden Delicious’ apple. Journal of Horticultural Science 67:465-480.

Schupp, J.R., D. Straub, D.A. Rosenberger, and C.B. Watkins. 2001. Managing ‘Honeycrisp’ for production and quality. Compact Fruit Tree 34:107-109.