General Introduction to Avocados
By Mary Lu Arpaia

The avocado (Persea americana Mill.) is a evergreen tree which is native to Mesoamerica. Its natural range covers diverse environmental conditions from the central highlands of Mexico where frosts occur to the rain forests of northwest Columbia (Smith et al., 1992). The trees are evergreen and may reach heights of up to 20m. The trees are shallow rooted and have poor water uptake and hydraulic conductance. Although the trees produce an abundance of flowers, usually less than 0.1% of the flowers set fruit and most of these fruit abscise within 6 weeks from full bloom (Whiley and Schaffer, 1994). Vegetative growth is cyclical with pronounced growth flushes. There may be one to six shoot flushes per year (Thorp, 1992). Axillary branching may be proleptic, that is, a shoot develops only after a period of dormancy as a resting bud or sylleptic, where shoot growth occurs simultaneously with the parent axis with no dormant phase (Hall et al., 1978). In avocados, it is quite easy to discern between these two types of axillary branching.

Three climatic factors are considered to influence flowering and fruit set in avocado: 1) the occurrence of frost during the winter; 2) low mean temperatures during flowering and fruit set; and 3) the occurrence of extreme high temperatures during fruit set (Hodgson, 1947). Studies by Inoue and Takahasi (1989) demonstrated that floral bud initiation occurs predominantly in November for the cultivars Zutano, Jalna and Fuerte which would partially explain why low winter temperatures would impact avocado flowering and fruit set. Sedgely (1977) demonstrated that low temperatures (< 10C) during bloom can negatively affect flowering and fruit set. Temperatures exceeding 35C has also been shown to negatively influence flowering, fruit set and fruit retention for cv. Hass (Sedgely and Annells, 1981).

Current annual production in California and elsewhere suffers fluctuations caused by periodic freezes, droughts, winds, or other factors including the alternate bearing habit of the crop (Wolstenholme and Whiley, 1992). The 25 year (1970-71 through 1995-96) average production in California has been 5713 lb/acre, with a high of 10,052 lb/acre in 1974-75 and a low of 2,709 lb/acre in 1971-72. In the last 25 years there has been 7 years where average production has exceeded 7,000 lb/acre. In each of these instances the following year's production declined an average of 41%. The great difference between crop volumes from year to year create serious financial challenges for growers and marketers alike. This costs the industry millions of dollars in lost markets during low crop years and in oversupply during heavy crop years. It is the goal of many international researchers to discover methods of sustaining annual avocado production and reducing alternate bearing. This goal is especially important to the California industry. In order to remain competitive both in the national and international market, it is important that tree productivity be increased so that the economic viability of the California industry can be maintained. The California avocado industry occupies a unique agricultural niche since it is cultivated at the urban agriculture interface. The crop is ideally suited to this niche since the industry is largely pesticide free. Most insects pests are controlled through the release of beneficial predators and avocado root rot, Phytophthora cinnamomi Rands., is managed through the use of tolerant rootstocks.