Influencing Fruit Finish on Yellow Skinned Cultivars

One of the most difficult things to manage in orchard culture is the look of the skin on varieties that have a tendency to roughen up and russet as the season progresses. This is in part related to a number of factors. The list of items involved in fruit russeting is somewhat endless. Primarily it is related to the cultivar itself and its inherent inability to wall off all those factors that can infect, disrupt, wound or otherwise prevent the apple from developing that award winning look that consumers pay for.

On the list of russet causing factors are things like mildew infection, frost, yeast infections, high heat and UV, copper compounds applied at the wrong time, excessive wetting, inappropriate combinations or mixes of chemicals and so on. What is consistent form year to year is the influence of the spring weather on the % cullage in varieties like Golden Delicious.

We know from experience that a cool wet spring will produce more russeting than a warm dry spring. Some varieties of apple have a more absorptive skin and water that is left on the skin during the cell division stage of growth can absorb and burst epidermal cells under the developing cuticle or waxy surface leaving a lot of wound tissue that never heals smoothly.

One of the harshest treatment regimes that can be used on apple while it is growing is the copper family of compounds. Applied at the wrong time of the growing season, one is guaranteed to produce symptoms of russet. Straight copper sulphate or even fixed copper on a susceptible cultivar after growth has started in the spring can produce disastrous results. The copper family of compounds is also indispensable on crops such as sweet cherry during the dormant season for control of bacterial diseases. This is also the same material that is useful in defoliating apple nursery stock in the fall of the year.

One of the projects I have been involved with for several years now is the finish of cultivars like Crispin, Golden Delicious and now Aurora Golden Gala. I have had some very promising results from the use of specific treatment regimes that are based on the use of particle film technology.

This past season I was paying particular attention to a group of Aurora trees that I was treating with Promalin and Surround Crop Protectant particle film.

The past growing season was extremely hot with very high UV indices through the growing season. Typically, during the midday hours, an apple sitting in full sunlight would be at least 20 degrees warmer on the sunny side of the apple than on the inside of the fruit. To the touch, the fruit would feel warm on the outside while the inside would feel cool. In order to cope with these types of temperature differences an apple produces a different lenticel structure and will move more water through that side of the apple to keep from burning up. This is where the particle film comes in. Temperatures are moderated and lenticels do not get that prominent and coarse look.

Figure 2 - Promalin/Surround treated trees of Aurora Golden Gala at harvest in late September. Fruit size averaged more than 180 grams for treated fruit.

Treatments of Surround were applied from mid June to mid August after a 3-spray regime of Promalin starting at king bloom petal fall.

At harvest, fruit from treated trees were compared to fruit from untreated trees. What was outstanding was the difference in finish of the fruit. Treated trees produced a lemon yellow coloured fruit with a silky smooth finish and inconspicuous lenticels while untreated trees produced fruit with a rougher finish that was characterized by a much higher % of lenticel spotting and more of a greenish yellow colouring with a prominent and bronzy orange wash on the hot side of the apple.

Figure 3 - Fruit from treated trees in the top row compared to fruit from untreated trees in the bottom row. Fruit on trees from both treatment regimes were spray thinned and then hand thinned in mid June to 20 cm apart. Trees in both treatment regimes were irrigated.

Fruit size differences were not as noticeable as they were in the previous year in the absence of hand thinning. I would hypothesize that the particle film enabled treated trees in the previous year (2006) to carry and finish a heavier crop load of larger fruit due to greater net photosynthesis and more carbon being sent to the crop.