Low temperatures and fluctuations in environmental conditions are among what researcher Ashraf El-Kereamy described as a “possible hypothesis” for the cause of lemon pitting in California’s San Joaquin Valley. Other possible causes include what he called “overdoing a good thing” such as fertilizer and irrigation, or something affecting the integrity of the lemon’s wax layers. El-Kereamy is director of the University of California Agriculture and Natural Resources Lindcove Research and Extension Center and chief investigator for the relatively new lemon pitting issue.
In a survey this summer, valley lemon growers indicated they first observed lemon pitting in 2015. The disorder was observed in other orchards starting in 2020. It has appeared on Lisbon lemon, one of California’s most widely grown lemons. Lemons on trees grown on various rootstocks and various types of soils, from 4 to 70 years old, have been affected.
Lemon pitting damage starts to appear in the first week of May as a darker area on the rind, El-Kereamy said. Most of the damaged fruit have rough rinds. “Low temperature at early stages increases cell division, probably causing the rough rind,” El-Kereamy said.
Although researchers aren’t sure what causes lemon pitting, some things have been ruled out. “It is not a spray-related damage, as it occurs in all groves, even before and after the petal fall,” a slide in El-Kereamy’s presentation stated. The same slide reported that tests did not show any pathogens, and that damage has started to appear in grapefruit, blood oranges and navel oranges.
During questioning following a formal presentation on Sept. 8, El-Kereamy said there is no correlation between lemon pitting and an insect. However, surveyed growers reported that thrips and two-spotted mites have been observed in some affected groves.
“We are working on collecting more data,” El-Kereamy stated.
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