THE

^)uring the past decade many diseases of:rop plants which have long been consid-

I ?red to be virus diseases, have beenI found to be caused by agents other thanLruses. The agents include mycoplasma-

organisms, viroids, and novel bac-Che discovery of such newly recog

nized causes of disease came about mainlythrough examination of diseased tissueswith the electron microscope. Methodsfor preparing sections of tissue suffi¬ciently thin to look at with the elec¬tron microscope were developed onlyduring the last twenty years. This ex¬

plains why these new disease-causingagents were overlooked for so long.

Mycoplasma-1ike organisms (MLO). Agroup of organisms called mycoplasmshave long been known to cause respira¬tory and other infections in man andother animals. These organisms areabout half as big as bacteria and, un¬like bacteria, they lack a rigid cellwall. They are surrounded only by athin membrane. They may be spherical,long and filamentous, or sprilli-form. The latter have been named

spiroplasmas. In plants, their growth,reproduction, and movement is restrictedto the phloem or food-conducting cells.Most MLO are carried from plant toplant by leafhoppers in which they canalso multiply. They typically inducestunting, yellowing, and various typesof malformations in plants. Symptomscan often be reduced or even elimi¬nated by treating plants with tetracy¬cline antibiotics. Examples of import¬ant diseases in California caused byMLO are aster yellows, pear decline.

peach western X disease, cherry buck¬skin, and citrus stubborn.

Viroids. Viroids are small, naked vi¬ruses. Normally viruses consist of one

or more strands of ribonucleic acid(RNA) packaged in a coat of protein.In contrast, viroids are only verysmall strands of RNA. The viroid RNAcarries very little genetic informa¬tion compared to viruses and yet it iscapable of causing disease, replicating,and being transmitted from plant toplant. Diseases caused by viroids are

potato spindle tuber, citrus exocortis,and chrysanthemum chlorotic mottle.

Bacteria. Although bacteria have longbeen known as the cause of many plantdiseases, only recently have they beenimplicated in diseases such as Pierce'sdisease of grapevines and alfalfa dwarf.These two diseases have long been thoughtto be caused by a virus because of thecontagious nature of the disease and thefact that no fungi or bacteria could beisolated from diseased plants. Recently,however, bacteria were isolated fromleafhoppers which were feeding on dis¬eased plants. The isolated bacteriawere then injected into clean insectswhich, in turn, fed on healthy plantsand produced the disease. The organismis gram-positive but not a member of thegenus Corynebacterium (the only gram-positive genus of plant pathogenic bac¬teria heretofore recognized). Other dis¬eases which may be caused by the sameor similar bacteria are ratoon stunt of

sugarcane, phony peach, and almond leafscorch.

CO-OPERATIVE EXTENSHJN WORK IN AGRICULTURE AND HOME ECONOMICS,U.S. Department of Agriculture, University of California and County of Kern Co-operating

2610 "M" Street, P.O. Box 2509, Bakersfield, California-93303 Telephone: (805) 861-2631The University of California's Co-Operative Extension programs are available to all, without regard to race, color or national origin.

Virus diseases characteristically are notcontrollable by treatment with therapeu¬tic chemicals. However, bacteria andMLO are sensitive to a wide range of

antibiotic materials. Thus, knowledgeof the true causes of these diseases

opens the way for more effective con¬trol measures in the field.

************

CALIFORNIA GRAPES: ESTIMATED PLANTINGS AND ,

REMOVALS, BY DISTRICT AND CLASS, 1975-76

District

Estimated

Plantings BearingREMOVALS

Non-Bearing Total

Wine Varieties

North Coast

Central Coast

North San JoaquinSouth San JoaquinOther

850140280650100

2,500

1,1806,700

200

250500

4,600200

2,750500

1,18011,300

400

Total 2,020 10,580- 5,550 16,130

Table Varieties

North San Joaquin — 300 — 300South San Joaquin 275 2,800 50 2,850Other — — — —

Total 275 3,100 50 3,150

Raisin Varieties

North San Joaquin — 200 — 200South S^ Joaquin 1,250 2,900 640 3,540Other — — — —

Total 1,250 3,100 64o 3,740

All Varieties

North Coast

Central Coast

North San JoaquinSouth San JoaquinOther

Total

850 2,500 250 2,750140 — 500 500280 1,680 — 1,680

2,175 12,400 5,290 17,690100 200 200 4oo

3,545 16,780 6,240 23,020

1/

2/

3/

Estimates are based on a survey of the University of California Extension FarmAdvisors located in the various grape growing counties, from their assessmentsin January 1976.

Not estimated.

Incomplete _2-