Entomology, Plant Pathology and Weed Science, New Mexico State University, Las Cruces, New Mexico 88003, USA

Background and objectives
Phytophthora root rot (PRR) caused by the soilborne pathogen Phytophthora capsici has been described as a serious problem on peppers worldwide, and is the most important fungal disease that pepper growers face in the USA. The pepper variety Criollo de Morelos-334 (CM-334) has been reported as the source with the highest resistance to PRR. However, few studies have been conducted to understand the host-pathogen interactions (Capsicum annuum/P. capsici) in this pathosystem and the mechanism of resistance in CM-334. Therefore, the goal of this research was to gain a better understanding of this pathosystem as compared with the commercial susceptible pepper, New Mexico 6-4 (NM 6-4). The objectives of this study were: to examine the progress of Phytophthora capsici in the roots of NM 6-4 and CM-334 and determine at which stage of the fungus life cycle resistance is effective, and to determine whether resistance is due to compounds formed in the foliage and/or roots.

Results and conclusions
Grafting experiments showed that resistance to P. capsici causing PRR in CM-334 is due to compounds formed in the roots. No differences in zoospore attachment, germination or penetration were observed between resistant and susceptible pepper roots. The resistance mechanism appeared to be effective during colonization when fungal growth and reproduction were reduced in CM-334 compared to NM 6-4. A reduction in P. capsici recovery was observed in CM-334 with time after inoculation, but the fungus was still isolated for up to 75 days. These results demonstrate that resistance is effective after penetration and is fungistatic rather than fungicidal.

The hypothesis that resistance is fungistatic and of biochemical nature was tested by studying several compounds that may play a role in the defence response. Total RNA Northern analyses from roots showed a slight induction of PAL (phenylalanine ammonia-lyase) in CM-334 earlier than in NM 6-4. Therefore up-regulation of other enzymes in the phenylpropanoid pathway, related with the synthesis of phenolics that appear to be part of the defence response, need to be studied. A higher induction of HMGR (3-hydroxy-3-methylglutaryl CoA reductase, clone hmg2) was observed in NM 6-4 compared to CM-334. The induction of HMGR in NM 6-4 is probably correlated with wounds caused by the fungus. Peroxidase activity was significantly higher in CM-334 root tips after inoculation. Acidic peroxidases were observed in NM 6-4 and CM-334. These isoperoxidases increased in CM-334 after inoculation, suggesting a correlation with the defence response. Different phenolic compounds were observed with thin-layer chromatography in CM-334 and NM 6-4. A strong inhibition of P. capsici growth was detected with phenolic compounds from CM-334 roots. Our results suggest that phenolics and acidic peroxidases might be involved in the resistance mechanism in CM-334. It has been previously proposed that these compounds are correlated with resistance in the pepper genotype Smith-5 [1, 2].

1. Alcazar MD, Egea C, Espin A, Candela ME, 1995. Physiologia Plantarum 94, 736-742.
2. Candela ME, Alcazar MD, Espin A et al., 1995. Plant Pathology 44, 116-123.