PEACH Rots and How Temperature Affects Their Development

High temperatures favor the development of peach rots, yet summer weather may be too hot for the greatest growth, and the effect of a particular temperature will vary with the rot organism and with its stage of development.

Practically all peach rots are caused by one or the other of two decay organisms, Monilia and Rhizopus. Monilia causes the brown rot so common in orchards and on the market, and Rhizopus a soft watery rot of the ripe harvested fruit. Monilia grows best at 70° to 80° F. and makes practically no growth as the temperature approaches 90°, while Rhizopus grows best at 90° to 95°. Monilia rots can start and grow at as low a temperature as 32°, while Rhizopus rots that are already started can barely enlarge at 40°, and their early stages of development are almost completely inhibited at 50°.

At 41° Monilia rots develop about four times as fast as at 32°; at 50° about two and two-tenths times as fast as at 41° and practically nine times as fast as at 32°; at 59° about one and six-tenths times as fast as at 50°, three and five-tenths times as fast as at 41°,and fourteen times as fast as at 32°; at 68° about one and six-tenths times as fast as at 50°, two and five-tenths times as fast as at 50°, five and five-tenths times as fast as at 41°, and twenty-two times as fast as at 32°. They develop slightly faster at 77° than at 68°, but at higher temperatures the growth soon begins to fall off.

Rhizopus rots rarely make a start at 50° F., but can grow fairly well at that temperature when once established. They develop about three times as fast at 59° as at 50°, about one and eight-tenths times as fast at 63° as at 59°, about one and five-tenths times as fast at 77° as at 68°, and about one and two-tenths times as fast at 86° as at 77°.

These various ratios apply approximately both to the number of rots appearing at the different temperatures and to the rate of development of a particular rot. They are quite significant in a study of the spoilage of harvested fruit. Peaches held in an ordinary refrigerator at 55° F. will develop less than half the Monilia rot that they would develop if exposed to the usual summer temperatures of 70° to 85°, and if held in a refrigerator at 40° they develop only about one-sixth as much rot as if exposed to outside temperature. Similar cooling of peaches to 55° reduces the Rhizopus rot to about one-fourth or one- fifth of what it would be at the usual summer temperatures, and holding at 40° entirely prevents its start.

Peaches held over in the packing house for the large part of a day before loading for shipment may develop as much tendency to rot in that time as during the whole period of transit under refrigeration.

If loaded warm, the fruit in the top of a four-layer refrigerator car is usually 12° to 25° F. warmer than that in the bottom of the car during the first 24 to 36 hours after loading, and during this time the Monilia rots in the top of the car are developing three to four times as fast as those in the bottom of the car, and the contrast with the Rhizopus rots is several times greater than this. The result is that the fruit in the bottom of the car may arrive at the destination practically free from rot, while that in the top of the car is often badly specked at unloading and ready for further rot development with any delay on the market or in the home.