Winter 1996 (v8n1)
	Urban and agricultural wastes for use as mulches on avocado and citrus and for delivery of microbial biocontrol agents. W.L. Casale, V. Minassian, J.A. Menge, Carol J. Lovatt, Elinor Pond, E. Johnson, and F. Guillemet Journal of Horticultural Science 70:315-332. 1995 The use of mulches may be beneficial for citrus and avocado crops, since they are very shallow-rooted. Some mulches have been shown to reduce Phytophthora root rots through one or more of several mechanisms. This multidisciplinary project, conducted at UC Riverside, examined the use of various organic materials as mulches on young citrus and avocado trees. The goal of the research was to determine the effects of these materials on plant growth and health and their ability to support the growth of microbial biocontrol agents. The researchers also correlated biological parameters with chemical composition of the mulches to determine which mulch characteristics support growth of the trees and of microbial biocontrol agents. Field experiments are also underway to complement this greenhouse work. Materials and Methods Eighteen mulching materials were used in the experiment, some of which were composted. Mulches were applied to 'Topa Topa' avocado and 'Troyer' citrange seedlings recently transplanted to pots in a greenhouse (500 ml of mulch per 3-liter pot). Five replicate plants each were used, and the experiments were terminated after three months. Analyses were made of numerous characteristics of the mulch materials, growth of three microbial antagonists of pathogens, and various plant responses. Results The greatest amounts of ammonia were released from grass clippings, milled almond and peanut hulls, and alfalfa hay (Table 1). None of the mulches significantly increased shoot growth of citrange seedlings compared to the control. In contrast, avocado shoot growth significantly increased with mulches of yard waste, grass clippings, and earthworm-composted sludge. Almond and peanut hulls were clearly detrimental to the roots of both citrange and avocado roots and to the shoot growth of avocado. The microbial biocontrol agents as a group grew best in yard waste, sudangrass hay, wood compost, rice hulls and rice hulls/paper. Of these, only yard waste, rice hulls, and rice hulls/paper were completely acceptable as mulches for avocado and citrus. Discussion Citrus and avocado are well suited to benefit from mulches. They have very shallow root systems, with the far majority of their roots located in the top 1 to 2 feet of soil. Avocado roots will often grow up directly into a mulch layer; this fact led the researchers to believe that avocado responds more positively to mulching than does citrus. Among the mulches examined, the structure and chemical composition of yard waste most closely resembles a forest litter layer similar to that in which citrus and avocado evolved. According to the authors, the beneficial effects of yard wastes can be explained by the high-nitrogen grass that offset the high carbon-to-nitrogen composition of wood chips and greatly reduced any temporary nitrogen shortage. The authors believe that citrus responded better with higher nitrogen and lower carbon mulches than did avocado because citrus responds to nitrogen deficiencies far more rapidly than does avocado. Some of the mulches, such as almond and peanut hulls, several manures, and alfalfa hay, had negative effects in this experiment: They reduced shoot and/or root growth, released large amounts of ammonia upon degradation, and resulted in undetectable populations of at least two of the three biocontrol agents tested. Animal manures are among the most common amendments applied to citrus and avocado orchards. The authors noted that root damage after application of high ammonia-releasing manures is common, especially to young trees. (Well-composted animal manures which do not release ammonia are believed by the authors to be acceptable for use on citrus.) Young trees are also more sensitive to Phytophthora infection. The authors believe that young trees are more susceptible to damage due to the higher foliage-to-root ratio of young compared to older trees; also, older trees may not show damage from the use of manures even though some root loss occurs. They also caution against extrapolating the results of these greenhouse experiments to effects of mulching in the field. The successful growth of the three microbial biocontrol agents on several mulching materials was promising. Strains of the three species tested, the common soil bacterium, Pseudomonas fluorescens and the ubiquitous soil fungi, Gliocladium virens and Trichoderma harzianum, have showed potential for suppressing Phytophthora root rots of avocado and citrus in previous greehnouse experiments. The results of this research demonstrate several important points, including: 1) the excellent potential for some mulching materials, such as yard waste, for improving growth of trees; 2) mulches most beneficial to citrus and avocado roots are also efficient substrates for some biocontrol agents; and 3) a huge urban problem can be at least partially solved by diverting part of the waste stream to citrus and avocado orchards. For more information write to: W.L. Casale, Department of Plant Pathology, University of California, Riverside, CA 92521-0122. Table 1. Total ammonia released from mulches during 20 days of decomposition of mulch materials. Mulch Material Total Ammonia Released (g/g dry wt) Grass clippings 13,404 Milled almond hulls 10,943 Alfalfa hay 8,486 Milled peanut hulls 2,546 Composted sewage sludge 1,121 Chicken manure 943 Horse/cow manure 923 Sudangrass hay 706 Cow manure 558 Mushroom compost 546 Composted yard waste 545 Yard waste #1 182 Rice hulls 60 Rice hulls/paper 32 Earthworm-composted sludge 20 Wood compost 18 Orange peels 8 Soil (Maddock) 0.3 Yard waste #2 0. 2 (CI-CIT.221) Contributed by Chuck Ingels [ Back | Search | Feedback ]