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Entry Points for Fruit Infections

It is always quite surprising just how many different disease organisms can infect fruit in the pre- and postharvest period long after the battles with scab are done in the orchard. The problem with identifying the species of fungus responsible for these fruit rots is that a lot of the infections look very similar. As fruit matures on the tree it has an increasing ability to resist infection of some of the more common disease of apple like scab. However, other disease organisms like some species of Botryosphaeria can infect by spores germinating in and around lenticel cavities commonly found in the skin of the apple.

Other diseases, including mildew, can express their ability to infect the skin of apple during the last 2-3 weeks before picking. If it happens to be rainy and wet during those last weeks the fruit is on the tree, then conditions can be perfect for a variety of fungi to infect by way of mature lenticel cavities in the skin of the fruit. This is one way that various infections like bull’s eye rot can get started. This infection is what I call "lenticel centered". Golden Delicious and varieties with Golden Delicious in their background are quite susceptible to lenticel centered infections.

These lenticel-centered infections can be as problematic as those infections that get started by other entry points on the apple. These other entry points include puncture marks, unhealed stem ends and bruises on the surface of the apple. We normally think of the skin of the fruit as being very tight and waxy. In a greater sense, this is correct. However, the surface of the apple skin is dotted with lenticels.

To the human eye, these lenticels appear to be nothing more than a roughened dot on the surface of the skin. In reality, lenticels are a very distinct cavity that can have highly defined margins or very indistinct and rough fissure like corners and walls. These lenticels act as a portal of entry to the vast supplies of carbohydrates, carbon-based polymers and minerals contained in the flesh of the fruit. Below is a close-up of a lenticel showing its anatomical features and essentially its true character.

Lenticels function in many ways. They allow both water and gas exchange to take place while the fruit is actively growing on the tree. Without skin lenticels, the fruit would not survive a day in the orchard. These lenticels regulate such things as fruit temperature by allowing the escape of heat laden vascular moisture. We do also recognize that these same lenticels do offer a portal of entry for various fruit rotting disease organisms.