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Four 'Lisbon' lemon selections, 'Frost Nucellar', 'Corona Foothills', 'Limoneira 8A' and 'Prior' were selected for evaluation on Citrus volkameriana rootstock. 1994-2002 results indicate that the 'Limoneira 8A Lisbon' and 'Corona Foothills Lisbon' are superior in yield and fruit earliness.

An experiment was initiated in 2000 to study the feasibility of growing organic lemon in the southwest desert of Arizona. An eight-acre field was selected on Superstition sandy soil at the Mesa Agricultural Research Center to conduct this investigation. Lemon trees were planted at 25 feet spacing in 1998. The initial soil test in top 6 inches was 5 ppm nitrate-nitrogen and 4.9 ppm NaHCO3-P. Soil pH was 8.7 in the top 6 inches. Four treatments were applied in randomized complete block design repeated four times. The treatments were beef cattle feedlot manure and perfecta, clover and guano, guano and perfecta, and standard practice treatment. Soil samples were collected from 0-6 and 6-12 inches the first week of March 2001 and analyzed for NO₃-N NH₄-N, total nitrogen, organic matter and available P. Preliminary results showed no difference in NO₃-N, NH₄-N in 0 to 6 and 6 to 12 inches between treatments. Total nitrogen increased significantly from 0.0262% in standard treatment to 0.0375% in the manure treatment. Similarly, soil organic matter increased from 0.297% in standard treatment to 0.4337% in the manure perfecta treatment. Phosphorus level increased significantly from 6.962 ppm in guano perfecta to 11.187 PPM in manure perfecta treatment. Leaf tissue analysis indicated that nitrate level was influenced by treatment. Yields of Guano treatments were significantly greater than yields of the other treatments. Both commercial standard and organic treatments were equally effective in controlling citrus thrips, but repeated applications were required. Mite population has been detected at low level with no significant differences observed among treatments.

In a rootstock evaluation trial planted in 1993, five rootstocks, 'Carrizo' citrange, Citrus macrophylla, 'Rough Lemon', Swingle citrumelo and Citrus volkameriana were selected for evaluation using 'Limoneira 8A Lisbon' as the scion. 1994-2002 yield and packout results indicate that trees on C. macrophylla, C. volkameriana and 'Rough Lemon' are superior to those on other rootstocks in both growth and yield. C. macrophylla is outperforming C. volkameriana. For the second year in a row, 'Rough Lemon' trees performed similarly to C. macrophylla and better than C. volkameriana. 'Swingle' and 'Carrizo' are performing poorly. In two other rootstock evaluation trials, both planted in 1995, C. macrophylla and/or C. volkameriana are outperforming other trifoliate and trifoliate-hybrid rootstocks under test.

Three lemon cultivar selection trials are being conducted at the Yuma Mesa Agriculture Center in Somerton, AZ. Data from these trials suggest that 'Cavers Lisbon' and 'Limonero Fino 49' selections may be suitable alternatives for the varieties most commonly planted in Southwest Arizona today.

'Fairchild' mandarins in the Phoenix area and 'Lisbon' lemons in Yuma were girdled beginning in November 1996. November, March and May girdling of the mandarins led to the greatest yield the first year, while March and May girdling led to the greatest yield in years 2 and 3. March girdling yield increases were generally due to greater fruit numbers, while in May, yield increases were due to greater fruit numbers and fruit size. Returns per acre suggest that March and or May girdling of mandarins will lead to greater profits for the grower. Like mandarins, lemon yields were greater following November, or November and March girdling after one year of the experiment. However, yields of these trees dropped considerably the second year, and the trees appear to be in an alternate bearing cycle. No lemon girdling treatment appears to be better than the untreated trees after three years.

Kaolin (Surround) was highly effective at preventing citrus thrips populations from reaching damaging levels in Arizona lemons. Applications should be initiated before thrips become numerous. Applying the material before petal fall may offer protection of early set fruit, but may not be necessary if thrips densities are low. However, since kaolin should be applied in advance of thrips populations increase, determining the benefits of pre-petal fall applications of kaolin is difficult. Kaolin applied on a maintenance schedule offers continual suppression of thrips populations, whereas traditional standard insecticides offer temporary population knockdown. Kaolin did not interfere with photosynthesis or stomatal conductance, and may possess yield enhancement qualities.

Trisert CB replaced conventional foliar applied low-biuret urea and liquid urea ammonium nitrate in a typical N fertilization regime, a urea triazone based N source. There was no yield decrease, change in fruit size or grade with the use of the Trisert CB. There were no differences in leaf P, K, Ca, Mg, Cu, Fe, Mn or Zn concentration. Occasionally, leaf N concentration of trees supplied with foliar applied Trisert CB was higher than that of the control treatment.

This experiment was initiated in March 2000 to study the feasibility of growing organic lemon in the desert southwest of Arizona. A ten-acre field planted to lemons in 1998 was selected on Superstition sand at the Yuma Mesa Agricultural Research Center. The initial soil test in top 6 inches was 5 parts per million (ppm) NO₃⁻ and 4.9-PPM NaHCO₃⁻-extractable P. Soil pH was 8.7 in the top 6 inches. Seven treatments were applied in randomized complete block design repeated three times. The treatments were control, compost and clover, compost and perfecta, compost and steam, manure and clover, manure and perfecta and manure and steam Leaf tissue analysis indicated that nitrate level was significantly influenced by treatment. Organic insect control treatments for citrus thrips were as equally effective as the non-organic commercial standards.

The residual activity of insecticides to second instar citrus thrips was measured on lemon foliage in 1998. In April, Dimethoate and Agri-Mek provided only knockdown control of thrips, dropping to <70% mortality by 3 days after treatment (DAT). Baythroid performed slightly better, providing 95% mortality 3 DAT, but by 7 DAT was giving about 74% mortality. Alert, Carzol, Success and AZEXP-2 provided the longest residual activity, lasting 7 days, but began to slip at 14 DAT. AZEXP-1 induced only 74% 0 DAT. Although, residual activity in general was greater in June than April, however this increase in residual activity did not necessarily increase the length of commercially acceptable residual. Agri-Mek and Dimethoate still only provided knockdown activity, and Baythroid was still giving 3 days of good activity. AZEXP-1 performed much better following the June application relative to the April application, providing 3 days of adequate activity. Although we are not certain the reason for this result, it maybe due to the adsorption properties of this chemical relative to leaf physiology. Alert performed similarly in June and April, and Carzol, Success and AZEXP-2 each lasted about 1 week longer.