'Hass' Avocado Tree Growth on Four Rootstocks in California. II. Shoot and Root Growth

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We investigated the relationship between alternate cropping and shoot and root growth in mature ‘Hass’ avocado (Persea americana Mill.) trees growing on ‘Thomas’, ‘Topa Topa’, ‘Duke 7’, or ‘D9’ rootstocks over five years in Southern California. Shoot growth occurred during two distinct flushes each year: one in spring and one in late summer. Root growth occurred throughout the year, with higher rates during spring and summer but slower growth rates during shoot growth flushes. There was no effect of rootstock on shoot growth rate. There was no direct relationship between alternate bearing and shoot and root growth. It is possible that cropping affected shoot growth, with extensive shoot development occurring in a year
with virtually no crop. The summer growth flush accounted for a greater proportion of the total shoot growth in years with light crop loads than in years with heavy crop loads. There were differences in root growth rate among rootstocks, but these relative differences varied among years. Root growth did not exhibit dormant periods as shoot growth did, but in general, root growth rate was greatest when soil temperatures were high and when shoots were not growing. This information gives us insight into the relative timing and relationships among growth events of avocado in Southern California and will help growers determine the optimal timing of cultural practices. Our results suggest that the best timing for fertilizing, spraying, and pruning is similar in trees growing on different rootstocks.

Michael V. Mickelbart
Paul W. Robinson
Guy Witney
Mary Lu Arpaia
Scientia Horticulturae