Tom Volk's Fungus of the Month for September 2002

This month's fungus is Venturia inaequalis, cause of apple scab

For the rest of my pages on fungi, please click

apple scab, caused by Venturia inaequalis-- photo by Debby Hanmer of UW-La Crosse

It's autumn in the United States, and that means back-to-school for millions of students. If you want to get on your teacher's good side right away, bring her or him an apple! But if you want to make a good impression, maybe you don't want to bring apples that look like those on the left. These apples are infected by the fungus Venturia inaequalis, and the disease is called apple scab. However, although they might look unappealing to you, these apples are perfectly good for eating, tasting exactly the same and with the same texture as uninfected apples, without any toxin production at all. Every year I go across the river to the apple orchards where I can get a large brown grocery bag full of infected apples for about US $5. I can also get beautiful uninfected apples at Applefest, held every September in La Crescent, Minnesota.

Venturia inaequalis actually grows only very superficially on the apple. The ascospores may infect the young leaves or the very young ovary in the flower, penetrating the cuticle. The infection remains outside the epidermis. eventually forming conidia (asexual spores) that push up the cuticle. The conidia are blown about by the wind, where they can infect new leaves and developing fruits. The cycle may be repeated throughout the summer, as long as conditions remain moist. Thus this infection cycle is said to be polycyclic. As winter approaches, the infected leaves fall to the ground. in the moister conditions near the ground, the mycelium of the fungus grows through the leaf. Any leaf that has been infected by more than one mating type of the fungus can be a site for sexual reproduction. V. inaequalis belongs to the Ascomycota; in this case asci are produced in pseudothecia (=pseudoperithecia), flask shaped structures shown to the left. The asci are said to be "bitunicate," meaning double-walled, which places these fungi in the class Loculoascomycetes. pseudothecium from Venturia on an apple leafThe asci produce ascospores (shown below) that are two-celled, with the cells being unequal in size-- hence the "inaequalis" part of the name. These ascospores are thick-walled, thus enabling the fungus to survive the winter. In the spring the ascospores are shed and can infect the newly developing parts of the plant, continuing the cycle.

Apple scab is a problem for growers in the northeastern and north central USA, where up to 70% of a crop can be lost. Although the apples are fine for eating, the yield is reduced, and sometimes leaves and other parts fall off the trees. Looking at the discussion of the disease cycle above, maybe you can already see an opportunity to control the amount of disease. Remember that the fungus overwinters as ascospores on the leaves and apples that have fallen to the ground and have overwintered. What do you think would happen if you raked up all the leaves and apples under the trees before the spring? There would certainly be much less inoculum to infect the trees in the spring, and the disease cycle would be broken. But what would happen if you raked up your leaves but your neighbors didn't? --and what if there were infected wild or escaped apples in the nearby woods? Since the ascospores are borne on the wind, in either case there would be plenty of inoculum to infect your trees and all your raking would be for naught. Ascospores could also be blown from miles away to start the disease cycle in your orchard.

So you turn to modern technology. There are also fungicides that can be sprayed to help prevent the spread of this disease. Now maybe you think all fungicides herbicides, and other pesticides are evil and harmful, and in many cases they are. If you've ever read Rachel Carson's "Silent Spring" you know something about how harmful pesticides can be if used improperly. However, judiciously used pesticides can be very effective and environmentally friendly. If you oppose the use of pesticides, maybe you should support organic growers.. More importantly, maybe you should become more tolerant of minor cosmetic imperfections in your fruits and vegetables. You should also be aware that some of the worst carcinogens and poisons are produced naturally by many kinds of plants and fungi, mostly to avoid being eaten by insects and other animals. Aflatoxin, produced by Aspergillus flavus and related species is known to be carcinogenic at one part per billion. The bitter tastes of celery and broccoli are due to chemicals produced to ward off herbivores. And think of that quintessential health food, alfalfa sprouts-- these contain the amino acid L-canavanine, an arginine analog that can be incorrectly inserted into proteins in your body causing malformation of your enzymes and other proteins. There are many other natural carcinogens, many of which are far more dangerous than pesticide residues. However, I digress.

closeup of two-celled ascospores in a pseudothecium In any case, I think it's safe to say that there is widespread agreement that pesticide use should never return to the indiscriminate use that occurred during the 1950's -1970's. Unfortunately it is near impossible to grow saleable apples without the use of fungicides. So the aim of our apple orchardist should be to minimize the use of fungicides. Besides the environmental cost, it would be economically cheaper to use less fungicide. But how to do this?

Remember that the disease is most easily spread if the conditions are cool and moist. What if the grower could figure out when to spray, based on the likelihood that the weather conditions would be conducive to the spread of the disease, either through conidia or ascospores? This is called disease forecasting, and there are computer programs commercially available that can combine weather data with what is known about the disease cycle. The orchard owners would only have to spray fungicides when the weather data and the computer program tell them to do so, thus cutting down on the amount of fungicide in the environment and a lower economic cost to the orchard.

A more long term solution is breeding resistant varieties of apples. However, it may take several years to get the resistance plus all the characters of a particular variety of apples. The major problem after that is that, through their sexual reproduction, Venturia can relatively quickly overcome this resistance.

Right across the Mississippi River from La Crosse is La Crescent, Minnesota, the apple growing capital of Minnesota. You can read more about the city and their fun Applefest held every September. This year's festival is September 19-22, 2002. I've become very fond of their apples, especially "Haralson," a local variety with delicious tartness and crispness. Try some if you can find it!

I hope you enjoyed learning something about Venturia inaequalis and fungicides today. Plant pathology is very complex in its science, as well as its economic and environmental aspects. To learn more about plant pathology see the website of the American Phytopathological Society. For more technical information on apple scab disease, please see this excellent site at Cornell University

Now go pick a scabby apple to bring to your teacher to teach your class about Venturia. Remember, the apples are still delicious!

If you have recommendations for future FOTM's please write to me at

If you have anything to add, or if you have corrections or comments, please write to me at

This page and other pages are © Copyright 2002 by Thomas J. Volk, University of Wisconsin-La Crosse.

Return to Tom Volk's Fungi Home Page