Fruits turn from green to yellow as they ripen. The fruit stem lacks a well-defined abscission layer, so fruit should be cut from the tree to avoid tearing the stem out of the flesh. Handle the fruit carefully - although hard, they bruise easily. Most cultivars start cropping at about 5 years of age. Don't store them near apples or other fruit as these will gain a quincey flavour.
Quinces are generally free of pests and diseases.
In severe cases, rake up and compost or burn fallen leaves in autumn. Bordeaux mixture can be sprayed when leaves open in spring. Fireblight - to reduce susceptibility, minimise pruning and fertiliser use. Hardwood cuttings in winter - easy. By seed where fruiting quality is not of great importance.