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Proceedings V World Avocado Congress (Actas V Congreso Mundial del Aguacate) 2003. pp. 719-724.

B.A. Faber1, A.J. Downer1 and J.A. Menge2
UCCE, 669 County Square Dr., Ventura, CA 93003, USA. E-mail: Plant Path, UC Riverside, CA 92527, USA. E-mail:

Organic mulches were applied to two avocado and twocitrus orchards over a four year period. Soil moisture, weed suppression, tree canopy, root growth, soil/leaf nutrient status and fungal populations were measured. There was a pronounced effect of mulches on soil moisture (reduced evaporative loss) and weed growth (substantially reduced with mulch), but plant canopy and leaf nutrient status changes were negligible during the time frame, althoughsoil nutritional changes were noted. An important effect of mulches on root architecture was an increase in root length and spatial distribution in avocado, which was not found in citrus. This change in rooting pattern may partially be responsible for improved disease resistance in avocado.

Key Words: Citrus sinensis, C. limon, Persea americana, Phytophthora, yard trimmings, soil moisture, soilnutrition

Citrus, with a value of more than US$600 million, and avocados, with a value of more than US$240 million annually, are among the 10 top most valuable commodities grown in California. In 1990, a law came into force which mandated the redirection of landscape trimmings from land fills to more beneficial use. The subtropical tree crops industry has been viewed as a potentialuser of mulch derived from diverted landscape materials. These crops have been targeted because of the relatively high value of the crop being able to pay for the hauling and spreading of the materials. The effects of yard trimmings as mulches have been shown to be highly beneficial to growth and production of numerous crops. The effects have ranged from reduced water use (Opitz, 1974), improvedwater infiltration (Gregoriou and Rajkumar, 1984; Jones, 1961), reduced weed growth (Lanini, 1988), increased soil fertility (Weeks, 1945), control of soil-borne diseases (Hoitink, 1991) along with myriad other effects (McNees, 1916). Avocado trees in particular may benefit from


V Congreso Mundial del Aguacate

mulches since they are prone to a devastating disease caused byPhytophthora cinnamomi – avocado root rot. This is a disease that is enhanced by soil and management conditions that exacerbate water logging. This disease affects about 75% of the California plantations. Citrus is also affected by 2 species of Phytophthora which, although not as debilitating as avocado root rot, affects nearly 100% of the citrus groves of Southern California. The purpose of this project wasto evaluate the effects of mulch on the growth of citrus and avocado and its effect on the respective soil fungal pathogens of these tree species.

The trial consisted of two lemon citrus orchards and two avocado orchards in coastal Ventura County, 150 km north of Los Angeles. Pommer Ranch consisted of 3 year-old ‘Eureka’ lemons on ‘Macrophylla’ rootstock planted on a sandyloam, 3 km from the coast. Twenty replicate trees received either a 2.5 cm layer of mulch, 7.5 or 15 cm initially and each year material was reapplied to the original depth. Twenty trees received no mulch. The mulched trees were treated annually with a raw yard trimming from 1994-98. Mulch was spread under the tree canopy out to the drip line and also down the rows as a strip, mulch was kept awayfrom the trunks. The trial was irrigated by drip irrigation and the treatments were randomized in 5 blocks (4 trees/treatment/block). A second citrus orchard (Essick Ranch) was laid out in a similar fashion. The trees were ‘Navel’ orange on ‘Troyer’ rootstock. The soil was a loam soil irrigated by microsprinkler. This site was at a higher elevation (800 m) than the Pommer site and was both...
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