Rootstock/Scion Combos: What Works and What Doesn’t

By Ernie Neff

For Phil Rucks and Tom Powers at Phillip Rucks Citrus Nursery in Frostproof, Florida, selecting rootstock and scion combinations starts by acknowledging that some varieties just don’t do well with HLB. “We don’t recommend some varieties regardless of rootstock,” Rucks says.

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These 8-year-old Vernia trees on Kuharske rootstock produce more than 400 boxes per acre.

Powers says varieties that are especially difficult to grow with HLB are Hamlin, midsweet and pineapple oranges, Murcott (honey tangerine), and virtually all grapefruit “with the possible exception of Star Ruby.”

On the other hand, Powers says many growers recognize that Valencia, OLL-4 and OLL-8 late oranges seem to tolerate HLB reasonably well. “The Sugar Belle mandarin is recognized as being one of the most tolerant varieties to HLB,” he adds.

Rucks says Valencia and Vernia, a midseason orange with Valencia-like qualities, “perform really well on rough lemon, US-802, US-942 and Kuharske Carrizo.”

Rucks adds that Sugar Belle works well with most rootstocks except Cleopatra, due to the high level of acidity which persists in the fruit.

According to Rucks, sour orange is still one of the best rootstocks for most varieties planted in high-salinity and high-pH soils like those found on Florida’s east coast. He adds that in 2017, UFR-4 seemed to tolerate salinity better than other rootstocks.

Old vs. New Rootstocks

Generations of Florida citrus growers knew a great deal about the rootstocks they selected well before planting them. For one thing, rootstocks weren’t released for use by researchers until their attributes had been evaluated for 10 years or more. Consequently, growers knew much about sour orange, Carrizo, Swingle, rough lemon and Cleopatra mandarin rootstocks.

That has changed a great deal since the discovery of HLB in Florida, say Phil Rucks and Tom Powers with Phillip Rucks Citrus Nursery. Growers needed new rootstocks to cope with HLB and were willing to assume the risk of planting rootstocks without long-term evaluation. The University of Florida Institute of Food and Agricultural Sciences and the U.S. Department of Agriculture have expedited the screening and release of several promising rootstocks over the past few years. Long-term evaluation will take place in the field.

“I think the (planting) site determines your rootstock selection most of the time,” Rucks says. In other words, pick a rootstock that works well in your soil and climate.

Rucks suggests some rootstock/scion combinations are best avoided. They include old-line Valencia clones on Cleopatra rootstock; Murcott on Kuharske Carrizo; early round orange varieties such as Hamlin on rough lemon and Volkamer lemon; and early Valencia varieties like Valquarius on rough lemon.

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About the Author
Ernie Neff

Ernie Neff

Senior Correspondent at Large