Infrequent Occurrence of Peach Skin Streaking and the Role of Rainwater Attributes on Symptom Development.

Streaks lacking pigmentation have impacted red blush cultivars of peaches in many East Coast production areas, but the underlying cause is still unclear. Some evidence suggests that streaking may be caused by reactive agents in rainwater. Peach skin streaking was monitored over two consecutive years at a commercial farm with a history of streaking problems located near Ridge Spring, SC. Six cultivars (two early season, two midseason, and two late season) were evaluated, each in two locations (LocA and LocB). Among those 12 experimental block cultivars, streaking occurred only in 2017 in cv. Scarletprince of LocA with an incidence of 6%. That same year two nearby nonexperimental blocks with 'Scarletprince' revealed 11 and 25% streaking. Streaking was also monitored at the Musser Fruit Research Center (MFRC) in Seneca, SC. At that location, a high incidence of streaking was observed, with 50 and 64% in 'Julyprince' (2017) and 'Carored' (2018), respectively. Rainwater pH taken from each of the 12 experimental blocks ranged from 3.03 to 7.4, ozone (O3) levels ranged from <0.02 to 0.37 mg/liter, and chlorine (Cl2) and chlorine dioxide (ClO2) levels were either just above or under the detection limit of 0.01 mg/liter and 0.02 mg/liter, respectively. Although the electrical conductivity (EC) was below 100 µS/cm on average, we did measure EC values as high as 1,500 µS/cm. For all samples, the oxidation-reduction potential (ORP) ranged from 90 to 302 mV, indicating oxidizing conditions. Fruit harvested 1 or 2 weeks prior to commercial maturity and treated with solutions of high (10) or low (3) pH, ozone >0.37 mg/liter, and EC values of up to 3,000 µS/cm did not produce symptoms. However, streaking was reproduced with collected rainwater, but the remaining sample volume did not allow further analyses. Using 0.05% ClO2 to induce streaking, we show that fruit of different cultivars varied in susceptibility when treated 1 week prior to commercial maturity, with 'Juneflame' being the most susceptible and 'August Lady' being the least susceptible. Our study shows that multiple factors determine the occurrence of streaking in peach orchards, including cultivar susceptibility, ripening stage, and the presence of rainwater with sufficient amounts of a yet unknown reactive agent or agent combination.