Three lemon cultivar selection trials are being conducted at the Yuma Mesa Agriculture Center in Somerton, AZ. Data from these trials suggest that 'Limonero Fino 49' and 'Cascade Eureka' selections may be a suitable alternative for the varieties most commonly planted in Southwest Arizona today. 'Femminello' and 'Villafranca' might also be planted on an experimental basis

Five rootstocks, 'Carrizo' citrange, Citrus macrophylla, Rough lemon, Swingle citrumelo and Citrus volkameriana were selected for evaluation using 'Limoneira 8A Lisbon' as the scion. 1998-99 results indicate that trees on C. macrophylla and C. volkameriana are superior to those on other rootstocks in both growth and yield. C. macrophylla is beginning to outperform C. volkameriana. 'Swingle’ and Carrizo' are performing poorly. In a similar trial, Four 'Lisbon' lemon selections, 'Frost Nucellar', 'Corona Foothills', 'Limoneira 8A' and 'Prior' were selected for evaluation on Citrus volkameriana rootstock. 1998-99 results indicate that the 'Limoneira 8A Lisbon' selection is superior, and that ‘Corona Foothills Lisbon’ may also be superior to the other selections in both growth and yield. Results from another lemon cultivar trial suggest that 'Cavers Lisbon', 'Limonero Fino 49' and 'Villafranca’ lemons may be good candidates for plantings as well. Results from two other lemon scion trials, a navel orange cultivar trial and a 'Valencia' orange trial are presented as well.

For 24 consecutive months, root pieces were collected from field -grown Lisbon lemon trees established on Citrus aurantium (sour orange), C. jambhiri (rough lemon), and C. volkameriana rootstocks. Root segments were wounded, inoculated with Phytophthora citrophthora or P. parasitica, and incubated for 96 hr in moist chambers. Smaller lesions developed during Jan -Feb than during Jul-Oct on root pieces of all tested rootstocks inoculated with P. citrophthora as well as root pieces of C aurantium inoculated with P. parasitica. Apparently there is a seasonal variation in the susceptibility of citrus rootstocks to colonization by Phytophthora. This information could be useful for more effective timing of fungicide applications.

Mature pecan trees at Picacho, Arizona were evaluated for variety performance during 1997-98. Total average yield per tree, percent kernel and percent viviparity were observed. During 1997, a severe viviparity year, only Bradley, Cheyenne, Souix and Tejas showed viviparity values of 20% or lower. Tejas did not return an acceptable percent kernel leaving Bradley, Cheyenne and Souix as potential varieties able to withstand low desert growing conditions on a regular basis. In 1998, the cool growing season confounded the test and no conclusions were drawn from the data.

The relationships between increasing nitrogen fertilization and growth, maximum CO₂ assimilation and the initial slope of the CO₂ response curve were studied in two ecotypes of wild strawberry, Fragaria chiloensis (L) Duchn. Nitrogen accumulation of CA11, an ecotype from a low -nutrient dune site, was greater at all nitrogen concentrations than that of RCP37, an ecotype from a higher- nutrient strand site. Maximum CO₂ assimilation, total Rubisco activity, dry weight, and initiation of leaves and crowns were higher in CA11 than RCP37 as nitrogen treatment was increased from 0 to 200 mg l⁻¹, whereas these parameters were lower in CA11 when fertilized at 300 mg l⁻¹, but not in RCP37. The mean leaf area of CA11 was greater than RCP37 when grown with no supplemental nitrogen, but mean leaf area of the two lines was similar under nitrogen fertilization. Maximum CO₂ assimilation and carboxylation efficiency increased with increasing leaf nitrogen in both clones. At equivalent concentrations of leaf nitrogen, RCP37 had higher CO₂ assimilation and carboxylation efficiency than CA11 and the difference between the 2 clones increased as leaf nitrogen increased. Thus, RCP37 had a higher photosynthetic nitrogen use efficiency than CA11. However, at a given applied nitrogen level, CA11 allocated more nitrogen to a unit of leaf area so that photosynthetic rates were higher than RCP37, except at the highest application of 300 mg l⁻¹. The high nitrogen accumulation capacity and resource allocation to fruiting structures (crowns) in CA11 lead us to suggest that this clone may possess genes that could increase fruit yield in cultivated strawberry.

Since 1973, Yuma County has had three apparently unrelated infestations of California Red Scale (CRS). The Yuma County citrus Pest Control District (YCCPCD) was successful in eradicating the first two in 1980 and 1984, respectively. We are continuing our spray program on the third, but our detection methods indicate that this infestation has now also been eradicated.