Red spot disorder
If you have ever seen red spots on peaches in SC, the usual culprit is scale. The scale insect sucks sap from the fruit (and growing shoots) causing red spots that often have a tiny white spot in the middle. At times, it may be possible to actually see the scale insect on the fruit using a hand-lens. If there are symptoms on fruit, it usually indicates a high infestation that is accompanied by symptoms on adjacent shoots including the presence of scales and perhaps signs of stress including wilting and premature leaf drop. If two dormant oil applications are made the previous winter, scale injury to fruit is unlikely.
A few growers in Anderson and Saluda counties had red spots on peach fruit in 2004
despite dormant applications of superior oil. At one site, red spots occurred on Calred peaches that were not typical of scale injury. Some differences included the fact that spots were smaller (1-3 mm) and they did not have a white center. The symptoms first appeared as very small red spots (1 mm or less) on green fruit before the yellow background or red blush color developed. As peaches grew, ripened, and developed their normal red blush, the spots largely disappeared because almost the entire surface of the peach became red (Calred is an almost solid red variety when fully ripe). The red spots on these fruit were not typical of either bacterial spot or scab. There was no evidence of any skin damage associated with the spots nor was there any evidence of damage below the skin. In addition, there was no evidence of scale damage or stress to the shoots of the affected trees. The disorder only appears on late ripening varieties and was also observed on Cresthaven, Georgia Bell, and Summer Pearl.
The spots were most likely what we call a Red Spot Disorder. The disorder was first reported in the early 1990’s by Drs. Zehr and Miller, Clemson University but research did not come to any firm conclusion on what caused the spot development. Some evidence suggested that the fungal pathogen Alternaria may be associated with the disease. Applications of fungicides helped to reduce the disorder incidence significantly in a 1995 spray test, supporting the idea that it may be a pathological problem. The experiment indicated that two to three applications of Ziram 76W at 2 lb per acre reduced red spot incidence from 47% to only 4%. The sprays were applied three times at 2-week intervals beginning three weeks after pit hardening (late May to mid June). This experiment was conducted in a commercial peach orchard in Orangeburg, SC.
We still do not know what actually causes the disorder but in 2005 we added a few more pieces to the puzzle. The disorder has only been observed during wet years. No symptoms were reported during the dry years between 2000 and 2003, but red spots caused severe damage in years characterized by frequent rainfall during the summer such as in 2004 and 2005. The disorder seems to be specific to certain midseason varieties. Among the most severely affected varieties in 2005 were Winblo, Redglobe, Monroe, Cresthaven, Calred, and Flameprince. Curiously, the disease only caused damage in orchards older than 10 years. Nearby orchards with younger trees of the same variety often did not show symptoms.
Some possible causes can be eliminated based on observational data. We do not think the disorder is linked to skin damage by sand because we observed the symptoms in areas with sandy and clay soil. The disorder is not linked to copper damage because some orchards with heavy symptoms were not sprayed with copper-based products during the growing season. We could probably list ten ideas of what could possibly cause the disorder but all of them are based on speculations. Obviously, a lot more work needs to be done to figure this thing out.
This symptom of small red spots developing into large black spots was found on 'Big red' peaches in 2009 with a 5% incidence. There was no bacterial flow and no fungus was isolated after two isolation attempts. There was no rot associated with the spots, but the yellow flesh underneath the black spots was penetrated by a dark red color (note the red discoloration on the top right corner as revealed by cutting off a portion of the fruit). The cause of this rare skin disorder is still unknown. Picture provided by Meg Williamson, plant problem clinic.