Tree Fruit Leader, Vol. 4(1) Mar. 1995

Reasons and Cures for Russet

by John Wilton, Senior Consultant (Deciduous Fruits)

EDITORS NOTE:

Russet was a serious problem in 1994, especially to Golden Delicious in the South Okanagan. This article, from the October, 1991 edition of the Orchardist of New Zealand, provides an excellent summary. Keep in mind that New Zealand is in the southern hemisphere and their growing season is opposite to ours.

Now that the spraying season is underway and fruit set is occurring it is time to turn our attention to russet.

Over the years a considerable amount has been written about russet. While these writings describe the problem, outline the conditions under which russet is likely to occur, and give general advice on how to minimize the problem, so far no magic russet prevention treatment has come to light. However, by recognizing the factors likely to influence russet expression and taking care to minimize their effects its possible to manage the problem.

Causes of Russet

In simple terms russet represents the healing of physical injury which has damaged the fruit skin. Because there are many ways fruit skin injury can occur the situations which can lead to russet are also numerous.

As far as I can ascertain weather and growing conditions have the greatest influence on whether or not the fruit is likely to be russetted.

Stage of growth and fruit development are also very important. Most authorities define the russet sensitive period for apple as the period from pink to about four weeks after petal fall.

Within this period the time from petal fall to about three weeks later is particularly critical.

There are also instances reported where russet has been induced outside of this period; however, as a rule potentially very damaging treatments such as severe sunburn, hail or phytotoxic sprays are necessary to induce russet symptoms outside of the recognized "normal" russet injury period. The severe sunburn which occurred during hot weather in December last year in Hawkes Bay is one such example of russet being induced outside of the critical flowering/post flowering period.

Russet expression, being a response to injury, lags behind so it often does not become obvious until late December, or even January. This is when the corky cells produced in response to earlier injury become visible to the naked eye as the fruit swells and the injured areas grow with the sizing of the fruit.

Weather Conditions

Wet changeable slow drying conditions with high humidity are most conducive and generally associated with difficult russet seasons. Warm nights with high humidity are also believed to favour russet expression.

At the other extreme frost, or near frost conditions lead to fruit injury which pre-disposes the fruit to russet.

Periods of low humidity with cool, but not frosty nights and warm dry days in the post blossom period are associated with good fruit finish years.

Tree and Growing Conditions

Tree health and vigour also influences russet expression. Fruit on trees with weak foliage that are under water stress or have poor drainage usually exhibit greater russet expression than those which are not under stress.

Very strong growing trees with excess nitrogen fertilizer have also been reported to produce fruit with more russet than those with a good tree vigour balance and balanced fertilizer programme.

As a general rule spur types show a greater tendency towards russet expression than non-spur types. This is possibly because with spur types the fruit is less protected by foliage during the critical post blossom period than are those on non-spur types.

Similarly fruit produced on very dwarfing rootstocks often shows a greater tendency towards russet expression than that produced on more vigorous rootstocks.

At the other extreme fruit borne in dense trees which suffer slow drying problems is more likely to show russet problems than fruit on well managed open textured trees.

Cultivars

There is a very wide range among apple varieties towards russet expression. Of the varieties we grow, Cox's Orange, the Gala group, Golden Delicious, Fuji, Splendour and Sturmer apples tend to russet readily. The Delicious group, Red Dougherty and Granny Smith have moderate sensitivity, whereas Coromandel Red and Braeburn appear to be fairly tolerant towards russet. I do not know where the newer varieties fit into the picture as I have not seen enough of them yet.

Recognizing cultivar susceptibility and planning your spraying practices around it will assist in managing the russet problem.

Spraying Practices

It's widely recognized that spray injury is a major factor in fruit finish problems. My own view is that the weather and growing condition factors already discussed probably have a much greater influence on russet expression than the modern pesticides we use these days are likely to have. This does not mean that we can, or should overlook the potential russet injury that spraying could cause.

Far from it, the sprays you choose and the way in which you apply them represents the major opportunity you have in managing your way around potential russet problems. Recognizing weather conditions and events which may have pre-disposed the fruit to russet injury is an important part of the skill in managing the spray programme to minimize russet problems.

Be particularly careful when spraying after rain or frost conditions as it's at these times when the fruit is most likely to be injured.

Quick drying is the most important rule in the reduction of spray injury risks. Sprays applied under slow drying conditions invariably cause more injury than those applied under fast drying conditions.

Low volume spray application, preferably in the range of 500 to 750 I/ha will usually give better fruit finish than high volume sprays because of faster drying. Poorly calibrated low volume sprayers, however, which tend to over spray parts of the tree to similar wetting levels as dilute sprayers will be more damaging because of the spray concentration factor, so make sure the sprayer is well adjusted and does not leave zones of overspray and excessive deposit.

If you have to spray under marginal drying conditions prioritize your order of spraying by block and variety on the basis of their repsective russet injury risk. Spray exposed quick drying parts of the orchard with the least russet sensitive cultivars first, then as things dry out work your way towards the more sensitive cultivars and the more sheltered slower drying parts to the orchard. As a general rule valley floors, and those rows shaded by tall shelter belts are the parts of the orchard where drying will be slowest.

To facilitate fast drying the trees need to be dry at the time of spraying; and, during the critical russet injury period, plan to complete spray application well before night fall so that the spray is dry well before dew is likely to wet the trees.

Spray Combinations

As a general rule "Kiss" (keep it simple stupid) applies to spraying.

For applications being made between pink and four weeks after petal fall, try and keep the number of materials in the tank down to a minimum. Incidentally, as the risk of russet injury increases with frequency of spraying you should also try and minimize the number of spray applications made over this period. So think carefully about dropping a material out of the combination if its going to mean another pass in a day or two's time.

This approach, however, is probably not justified if the weather has been lousy, application conditions marginal and you must get something on for black spot. In this situation you are probably better to leave such things as carbaryl thinning sprays and the like out and apply them a few days later when conditions are better.

Over the years I have seen many instances of subtle differences in pesticide formulation leading to marked variation in the quality of fruit finish.

For this reason I would be rather cautious in changing pesticide brand or formulation without first checking its fruit finish track record or establishing a small trial block to verify its performance in regard to fruit finish.

Watch out for water quality problems too. There are numerous overseas reports of iron levels about 2 ppm in the spray water leading to increased russet. Some also mention high calcium content as causing problems. Some other metallic ions in free solution, particularly copper, and zinc will induce russet when applied to apples over the russet sensitive period. As it's not absolutely essential that trace element sprays, or calcium sprays, or calcium sprays be applied over the period between pink and four weeks after petal fall, in the interests of fruit finish it's probably wise to avoid applying such materials during the russet sensitive period.

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