Changes in skin properties are consequence but not cause of green spot in 'WA 38' apples.

Abstract

The green spot (GS) disorder impairs appearance and restricts 'WA 38' marketing. The objectives of the present study were to characterize GS in developing 'WA 38' fruit. Green halos around lenticels were the first GS symptoms visible at 47 days after full bloom (DAFB) (GHL stage). Green halos darkened with the onset of red coloring (stage GHGS). By 96 DAFB, GS symptoms began to turn brown and sink relative to the surrounding surface (BHL stage). The first macroscopically visible cracks appeared by 133 DAFB. Most GS (92.5%) were associated with lenticels. Frequency of GS decreased from the proximal stem end to the distal calyx end. The frequency of fruit with GHGS or BHL symptoms increased as fruit diameter increased (r2 = 0.36***, n = 66). Lenticels on bagged fruit did not develop GS. Thin sections through the skins within GHGS revealed coagulated, brownish cytoplasm and positive lignin staining. Epidermal cells were smaller, and cuticular ridges were thicker within GS symptoms than in peripheral asymptomatic peels. The frequency of cracked lenticels markedly increased from intact asymptomatic lenticels (OKL) to the final BHL stage (severe GS). The skin tension was highest for the GHGS and the BHL stages and lowest in bagged 'WA 38' (BL). Cuticle and wax mass per unit area were higher within a GS than outside of a GS area. Strain relaxation on wax extraction was largest for cuticles from GS. There was little relaxation on excision and isolation. Transpiration was significantly higher through skin segments with GHGS symptoms than those with OKL. In summary, skin characteristics within a GS on 'WA 38' resembled those of a severely strained fruit skin where the strain results from a localized restriction of surface growth. The skin characteristics of symptomatic apples must be considered a consequence rather than a cause of the GS disorder.