Significant differences could be found between the proportions of affected seedlings in families from different plants of a clonal variety and even from different inflorescences of single plants. Increased proportions of affected seedlings were observed from similar crosses repeated in successive years.
June Yellows is not transmitted by grafting and no insect vectors are known.
Inheritance cannot be interpreted readily on a mendelian basis, but the hypothesis of plasmagene control convincingly accounts for all aspects of heredity. The concentration of a plasmagene would be expected to vary from cell to cell and this fluctuation would give rise to the observed variation in symptom expression and variations in hereditary transmission of the disease.
Carrier plants do not show symptoms until the plasmagene has multiplied to a threshold concentration. Multiplication rate and threshold concentration are both influenced by the genotype of a carrier plant.
The June Yellows plasmagene may be a mutant cell constituent of strawberry but more probably it is an exogenous, virus-like particle. High transmission of the disease through pollen indicates that the plasmagene may exist in intimate association with cell nuclei.
Some implications of this theory are discussed in relation to strawberry breeding and growing.