Viroid IIa is something of a marvel. Introduced in the first two years of a citrus tree's development, it reduces tree size, improves yield and fruit size, and causes the tree to produce fruit earlier in its life all characteristics that would be welcomed by citrus producers.

Those are the preliminary results of research by viroid pioneer Joseph Semancik, professor and plant pathologist at UC Riverside. He is studying trees that carry a variety of viroids at the UC Lindcove Research and Extension Center near Exeter.

Viroids are not small viruses. A small transmissible nucleic acid, the viroid is a single RNA with no protein coat. It's presence in orange trees was discovered less than 20 years ago, however, scientists believe they've coursed through citrus phloem in California for more than a century. Although they were first detected as disease-causing agents, a number of viroids do not cause disease, including IIa.

Viroids can be found in a wide range of plant species, Semancik said. "Virtually every grapevine in the world has viroids," he said. However, they do not exist in animals.

To study the effect of certain viroids, Semancik budded Campbell Valencia' orange scions from the UC Riverside Citrus Clonal Protection Program that were free of all known pathogens to Rubidoux trifoliate orange seedlings. At the time of budding, the seedlings were also inoculated with single viroid isolates. After growing in a greenhouse for a year, they were planted in 1984 along Lindcove's eastern boundary.

"There was little effective dwarfing the first two years," Semancik said. "But after two to four years in the field, the miniaturizing kicks in."

Today, the 15-year-old trees with viroid IIa are strikingly smaller than identical viroid-free trees planted at the same time one row away.

Semancik theorizes the viroid stresses citrus slightly.

"Trees, like people, operate a little better under stress, but not a lot of stress," he said, accounting for the production benefits of viroid-inoculated trees.

This year, Semancik will also compare the effects of the Christmas-time freeze on the fruit produced by trees that carry the viroid with fruit from non-infected trees.

Semancik's study at Lindcove compared three viriods and found that the one labeled IIa had the most consistent positive effects. Another isolate, IIIb, reduced tree size by 50%, but also reduced fruit yield.

"All trees containing viroid displayed a significant reduction in tree size," he said. "Trees with IIa consistently out-yielded viroid-free trees by about 15%, even though canopy volume was reduced by 20%."

Dwarfed citrus trees offer commercial producers the potential benefits of reduced nutrient, pesticide, pruning and harvesting costs. Currently, the most successful dwarfing of citrus trees is accomplished using the dwarfing rootstock Flying Dragon. However, it produces trees that, at 5- to 6-feet-tall, are too small for commercial production. Another rootstock produces trees about three-quarters as high as full-size 18-foot-tall trees, still bigger than optimal. The viroid may be the key to producing the ideal 8-to 10-foot tree.

Jeannette Warnert
Public Information Representative
Ag and Natural Resources
UC Center
550 E. Shaw Avenue
Fresno, CA 93710