Effects of Pecan Waste Products on Soil pH and Soilborne Pathogens in the Arid Southwest.

Waste products from pecan, such as pecan husks and pecan shells, are abundantly generated in Doña Ana County of southern New Mexico. There is a growing interest among producers in the utilization of pecan waste products to improve soil quality and to increase disease suppression capacity of soils in the region. The challenges faced by producers in this region include high pH of soils, which affects nutrient availability and disease problems in vegetable production. A laboratory study was initiated to assess the effects of pecan shells (PSH) and pecan husks (PHU) on soil pH and the suppression of Phytophthora capsici, a soilborne pathogen of many vegetable crops. PSH and PHU were applied at two rates (22.4 Mg ha^-1 [R1] and 44.8 Mg ha^-1 [R2]) under two physical states (ground or crushed), and were tested in a sandy loam soil. Air-dried soils, passed through a 2-mm sieve, were amended with the different treatments and dispensed into plastic pots (1179 cm³). The soil was initially saturated for 12 hours and allowed to drain for another 12 hours, after which they were transferred to a growth chamber set at 28^oC/22^oC (day/night).  The pH of the soil was measured on weekly basis for 4 weeks. Then, soil in each pot was divided into two sub-samples and air dried. In one set of sub-samples, soil was placed in a round plastic pot (340 cm³) and a chile plant was transplanted into each pot. Three days after transplanting, the pot was inoculated with zoospores of P. capsici.  The plants were monitored for disease development. In the other set of sub-samples, soil was analyzed for pH using saturated paste method. Results showed that both PSH and PHU significantly reduced pH compared to the control treatment during the 4 weeks of incubation. While PHU led to reductions in pH early during incubation, the effect of PSH in reducing pH became much stronger at the end of the incubation period. Both rates of application of PHU were similarly effective in reducing soil pH. However, for the PSH treatment, pH was more reduced at R2 rate compared to R1. Ground PHU and crushed PHU samples were equally effective, while ground PSH samples were more effective than crushed PSH in reducing pH.  At two weeks after inoculation, severity of Phytophthora blight was significantly reduced by both PSH and PHU. Ground PSH (at both rates) was more effective in reducing disease severity consistently than crushed PSH. Reduction in disease severity was similar at both rates of ground PHU. In contrast, disease severity was more significantly reduced at R2 rate of crushed PHU compared to R1. This study demonstrates the potential of pecan by-products for the improvement of soil pH and suppression of Phytophthora blight in the irrigated arid southwest.