Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Over the last few weeks growers have given me damaged pears that at the time I said looked like brown marmorated stink bug damage. Pears on the outside had pits in them where it looked like the bug had fed at one time (Photo 1). When the fruit is cut open there is a brown spot deep inside the pear that looks like brown marmorated stink bug (BMSB) feeding damage (Photo 2). But this spot differs from BMSB feeding in a couple of ways. First it is extremely hard, it is almost impossible to get a knife through it. Almost all of the BMSB damaged pears and apples and other fruit where I have seen a brown spot may be pithy or mealy or firm, but it has never been really hard. Second no yeast was found associated with the spot. In almost every case of BMSB feeding resulting in a dark spot in the fruit we have found yeast. And thirdly there was no stylet wound found in the damaged areas. I have found that if the cut is made into the fruit to the brown spot that the brown spot will then swell in a short time and stick up from the surface of the cut fruit (Photo 2).
The most likely cause of the damaged pear fruit appears to be Stony Pit disease, but even this we are not entirely sure. One problem is that the causal agent of Pear Stony Pit has yet to be isolated, but it can be transmitted by grafting and therefore a virus seems to be the most likely suspect, but no one knows for sure. Insect vectors or infected seed have not been documented as transmission factors for the virus.
The symptoms usually start about three weeks after petal fall, when dark green spots form on the fruit. The areas around these spots continue to grow, but the spots themselves do not. This results in misshapen fruit with pits (Photo 1). Pits often become necrotic and the fruit beneath becomes hardened (Photo 2). If fruit is heavily pitted it may become so hard that it is difficult to cut with a knife. Cracking of the bark, stunting of trees and chlorotic vein-banding have also been reported. One of the very odd things about this “disease” is that symptoms on fruit vary from season to season as well as severity. Trees that show symptoms one year may have no pitted fruit the following years. This type of scenario resembles damage more from BMSB feeding than a virus disease, which makes the diagnosis that much more difficult.
The best management practice seems to be to select virus-free trees for planting. Some of the most severely infected cultivars include Bosc, Comice, and Seckel, while less pronounced symptoms are found on Hardy, Conference, Forelle, Howell, Old Home, Packham’s Triumph, Bartlett, and a few other cultivars.
This is NOT MEANT TO SAY BMSB has not caused severe damage in the mid-Atlantic on fruit and vegetables because it has, just that fruit damage can be attributed to BMSB (as I did) when it was not the causal agent. Stony Pit seems to be fairly common this year in pears and any damaged-pitted fruit should be examined carefully.
Photo 1. Pear fruit with deep dimple in center caused by Stony Pit disease
Photo 2. Dimple area cut open showing extremely hard, raised brown spot