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 windo ws. A West Indian cultivar, 'Simmonds, and a Gualemalan x West Indian cultivar,'Lula,' were tested as scions on seedling rootstocks of the W est Indian cultivar 'Waldin.' Growth was calculated as linear extensio n rates (mm day(-1)). Although growth rates of individual shoots and r oots varied considerably within trees, flushes of shoot and root growt h were evident when mean rates of shoot and root growth were plotted o ver time. Flushes of shoot and root growth appeared to be synchronized and alternated on 30- to 60-day cycles. Shoot growth virtually stoppe d 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 cont rol of phytophthora root rot of avocado (caused by Phytophthora cinnam omi with fungicides. Since flushes of root growth in avocado alternate with flushes of shoot growth, shoot growth flushes could be used to p redict the peak occurrence of the susceptible actively growing root ti ps.