PERIODICITY OF SHOOT AND ROOT-GROWTH IN GRAFTED AVOCADO

Shoot and root growth of grafted avocado (Persea americana Mill.) was monitored in rhizotrons constructed of large pots and plexiglass windows. A West Indian cultivar, 'Simmonds, and a Gualemalan x West Indian cultivar,'Lula,' were tested as scions on seedling rootstocks of the West Indian cultivar 'Waldin.' Growth was calculated as linear extension rates (mm day(-1)). Although growth rates of individual shoots and roots varied considerably within trees, flushes of shoot and root growth were evident when mean rates of shoot and root growth were plotted over time. Flushes of shoot and root growth appeared to be synchronized and alternated on 30- to 60-day cycles. Shoot growth virtually stopped during the late fall and winter, but root growth, although it slowed to about one third the maximum rates during these seasons, continued throughout the year. The present information has relevance to the control of phytophthora root rot of avocado (caused by Phytophthora cinnamomi with fungicides. Since flushes of root growth in avocado alternate with flushes of shoot growth, shoot growth flushes could be used to predict the peak occurrence of the susceptible actively growing root tips.