Small disease particles known as viroids have infected San Joaquin Valley citrus trees for more than 100 years, often going undetected but sometimes stunting the growth of trees on trifoliate orange rootstocks. Local growers will learn of recent research on the problem from viroid pioneer Joseph Semancik, professor and plant pathologist at UC Riverside at a 1 p.m. field day Feb. 23.

About a dozen viroids with varying degrees of severity infect citrus. Survey's have found that Washington navel orange had a mild viroid since being imported to California from Bahia, Brazil, around 1875.

A small transmissible nucleic acid, the viroid is a single RNA that doesn't have the protein coat found on much larger viruses. It is transmitted by contaminated pruning equipment and bud wood used to propagate new trees. Semancik will discuss viroids' effect on valencia oranges at the February field day.

Also at the event, Staff Research Associate Raul Gonzales, who is based at the Lindcove Research and Extension Center, will review citrus planting techniques.

"It takes nurseries two years to grow a citrus tree," Gonzales said. "But if it's not handled correctly, it can be destroyed in two minutes."

Gonzales will cover the planting of sleeve trees, which come directly from greenhouses; ball trees, which are grown outside; and cylinder trees, grown in long, plastic pots. While most farmers have experience planting trees, Gonzales will share techniques with the best record of success.

The final speaker is UC Riverside Plant Pathologist Karl Steddom, who will outline biological control of phythophthora and replanting techniques.

The outdoor walking tour will be held rain or shine. The Lindcove Research and Extension Center is east of Visalia, off Highway 198. (Turn left off Highway 198 onto Mehrton Road, right on Rd. 228, then right on Ave. 316. Entrance is on the right.)

For more information contact Louis Whitendale, superintendent of the Lindcove REC, at (559)592-2408, llwhitendale@ucdavis.edu.

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