Answer: The leaves of your quince will turn yellow and then red in the fall so any yellowing this early in the season is most likely an indication of stress. Quince can also show signs of stress if over- or under-watered. There are some diseases that attack quince, and since you also see spotting, I'd consider that a possibility, along with cultural/environmental conditions. Full sunshine is perfect for your quince, as is a deep watering once each week. Quince does best in well draining soils so make sure the site drains well. Black spot, Fabraea maculata, is a common disease of quince can cause severe injury if not controlled. On the leaves the disease shows as small circular spots, reddish-brown on the periphery, with black centers. In the centers, numerous minute black spore-bearing pustules can be seen. A heavy infection will cause extensive defoliation. The causal fungus overwinters on the old leaves, producing two types of spores in early summer. These start new infections on the foliage and young fruit. Control can also be achieved with the use of fungicide sprays. Be sure to follow label directions; repeat sprays will be necessary.
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