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Fruitlet Mummification – A Puzzling and Costly Problem

One of the more outstanding issues around the process of thinning successfully is the residual number of fruitlets that sometimes hang onto the spur, shrivel and mummify. This is not serious in terms of drawing resources from the tree during the growing season. The problem comes in when these mummified fruitlets are colonized by the black rot fungi found in most Ontario orchards. The problem is chronic and costly in cultivars like “Gala”, “Fugi”, “Goldens” and “Idared”, as well as cultivars like “Northern Spy”. “Honeycrisp” is also a candidate because of its susceptibility to this disease.

Estimated losses in any one block of trees are serious, with the potential for 20-30% of the fruit on any given tree to be infected with black rot at harvest. What happens is that everything looks fine until about 2 weeks before harvest, when purplish spots appear on the fruit probably through lenticel infections. These infections are related to sporulation of the fungus on the mummy hanging directly above the maturing apple (see Figure 1).

Figure 1 - Failed abscission of Gala fruitlet directly above maturing apple is a potential site for black rot infection by way of colonization of the fruitlet by the black rot fungus as the fruitlet shrivels and dies.

To understand what happens during the thinning process, we have to look at the bloom fruit set period and the ensuing process through pollination and embryo development in the seed of the maturing fruitlet. As the fruitlet grows into an apple, the seed sends a signal to the rest of the spur by way of the stem tissue. These signals are controlled by the developing seed. The seed more or less tells the rest of the tree how to finish what was started during the bloom period.

During successful thinning, the developing fruitlet manages to get the message out to the stem to abscise and drop by presumably stimulating the formation of the abscission layer where the fruitlet is attached to the tree. If the thinning treatment kills out the developing seed in the fruitlet before those signals are complete, then the fruitlet remains attached and does not develop as would a fruitlet with living seed. These fruitlets eventually die on the spur and are easily colonized by the opportunistic black rot fungus Botryophaeria obtusa.

This year we are experimenting with different timings and treatments on “Gala” to determine if one or another treatment will produce more or less mummification with ensuing cullage from black rot infection. We are looking for the cleanliness of the thinning job, failed fruitlet abscission, mummification and black rot infection at harvest. The experiment has been designed using a variety of thinning treatments and timings ranging from petal fall through to 8 mm and onto a late timing of 12-14 mm king diameter. Thinning materials in the experiment include combinations of NAA, Sevin and Maxcel with the use of Regulaid. This experiment is being done in Middlesex County on a block of mature Royal Gala on M9.