A low mol. wt. RNA species is associated with sunblotch disease of avocados. The RNA (SB-RNA) was recovered from leaves and bark of symptomless carrier trees and from bark lesions of symptom-bearing trees. It was not detected in leaves or bark from lesion-free areas of symptom-bearing trees or in leaf or bark from healthy trees.

SB-RNA is soluble in LiCl and migrates in 20% polyacrylamide gels. In high salt buffers, it is resistant to ribonuclease A at a concentration of 1 µg/ml and is degraded only slowly at 10 µg/ml. The RNA is less resistant to ribonuclease A at low salt concentrations (0.01 ). The mobility of the SB-RNA in polyacrylamide gels is not affected by heat denaturation. The apparent mol. wt. of native SB-RNA is 65000 whilst under denaturing conditions it is 112000 to 115000.

SB-RNA occurs in high concentration in leaves of symptomless carrier trees being detected in 1 g of fresh leaf. In tissue fractionation experiments, SB-RNA was associated mainly with the chloroplast and endoplasmic reticulum fractions. It is concluded from these properties of SB-RNA that sunblotch is a viroid disease.


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