Heat Therapy and Stolon Apex Culture to Eliminate Mild Yellow-edge Virus from Hood Strawberry

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Abstract
Strawberry mild yellow-edge virus (SMYEV) was eliminated from 4-mm-long stolon apex explants of ''Hood'' strawberry plants held at a constant 38.degree. C in a growth chamber. Virus detection in each regenerated plant was by means of leaf graft analysis to a maximum of 3 indicator plants of Fragaria vesca. The percentage of virus elimination from these explants was directly related to the length of time they had grown at 38.degree. C. Equation I, which describes this linear relationship (Y = 70.35-1.69X in which Y = arcsin of the square root of the fraction of plants infected with SMYEV and X = number of days at 38.degree. C), predicts elimination of SMYEV from 50% of regenerated, 4-mm-long stolon apices excised from source plants after 15 days at 38.degree. C and 100% after 42 days. SMYEV was also eliminated from regenerated stolon apices 0.25-4 mm in length from infected Hood plants grown at greenhouse temperatures of 20.degree.-25.degree. C. Equation II, which describes this linear relationship (Y = 4.90 + 17.96Z in which Y = arcsin of the square root of the fraction of plants infected with SMYEV and Z = explant length in millimeters) predicts elimination of SMYEV from 50% of explanted shoot apices that were 2.25 mm in length and 98.6% that were 100 .mu.m in length. According to this equation, elimination of SMYEV does not occur from all explants from plants grown at ambient temperature regardless of their size. Heat therapy and stolon apex culture contribute independently to freeing explants of SMYEV and therefore can be combined to obtain increased reliability in developing strawberry clones free of SMYEV.

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