Powdery mildew (PM) is a very important disease of cucumber (Cucumis sativus L.). Resistant cultivars have been deployed in production for a long time, but the genetic mechanisms of PM resistance in cucumber are not well understood. A 3-year QTL mapping study of PM resistance was conducted with 132 F2:3 families derived from two cucumber inbred lines WI 2757 (resistant) and True Lemon (susceptible). A genetic map covering 610.4 cM in seven linkage groups was developed with 240 SSR marker loci. Multiple QTL mapping analysis of molecular marker data and disease index of the hypocotyl, cotyledon and true leaf for responses to PM inoculation identified six genomic regions in four chromosomes harboring QTL for PM resistance in WI 2757. Among the six QTL, pm1.1 and pm1.2 in chromosome 1 conferred leaf resistance. Minor QTL pm3.1 (chromosome 3) and pm4.1 (chromosome 4) contributed to disease susceptibility. The two major QTL, pm5.1 and pm5.2 were located in an interval of ~40 cM in chromosome 5 with each explaining 21.0-74.5 % phenotypic variations. Data presented herein support two recessively inherited, linked major QTL in chromosome 5 plus minor QTL in other chromosomes that control the PM resistance in WI 2757. The QTL pm5.2 for hypocotyl resistance plays the most important role in host resistance. Multiple observations in the same year revealed the importance of scoring time in the detection of PM resistance QTL. Results of this study provided new insights into phenotypic and genetic mechanisms of powdery mildew resistance in cucumber.

Maize (Zea mays L.) is grown on approximately eight million hectares of acidic soils, where yields are low because of the toxicity of AI and Mn and deficiency in Ca, Mg, and P. Maize cultivars tolerant to soil acidity would increase yields on such soils. A diailel study was conducted involving six soil‐acidity tolerant and two susceptible segregating populations to identify superior germplasm to develop cultivars for acidic soils. The eight populations and their 28 crosses were evaluated in seven acidic soil environments. Tolerant populations averaged higher in yield (2.19 vs. 1.58 Mg ha−1; P < 0.01), ears per plant (0.79 vs. 0.64; P < 0.05), and ear height (61.6 vs. 51.4 cm; P < 0.01), and fewer in days to silk (68.8 vs. 69.7 d; P < 0.05) than the susceptible populations. Mean squares of parents vs. crosses were highly significant for yield, ear height, and ears per plant, and significant for days to silk, indicating beterosis for these traits. Crosses between tolerant populations tended to yield higher (3.00 Mg ha−1) than those between tolerant and susceptible populations (2.40 Mg ha−1) and between susceptible populations (2.01 Mg ha−1). General combining ability (GCA) was highly significant for all traits, but specific combining ability (SCA) was significant only for ears per plant. Reciprocal recurrent selection would be effective in developing superior cultivars for acidic soils and should include populations 90SA‐3 and 90SA‐4 or CMS‐36 for yellow endosperm cultivars and 90SA‐6 and 90SA‐7 for white endosperm cultivars.

CIMMYT has developed a wide array of hard endosperm opaque‐2 (o2) maize (Zea mays L.) germplasm, through the combined use of two genetic systems involving the o2 gene for improving protein quality and the genetic modifiers of the o2 locus for improving kernel phenotype and ameliorating other o2‐associated defects. The objective of this study was to determine the heterotic patterns and combining ability of tropical quality protein maize (QPM) germplasm, and identify superior germplasm suitable for hybrid development. Ten parents (four QPM pools, five QPM populations, and experimental variety PR 7737) were used in a diallel study in eight environments. Data on grain yield, time to silk, plant height, and endosperm hardness were recorded. General combining ability (GCA) effects were highly significant for all traits; specific combining ability (SCA) effects were signififant for time to silk and plant height. Genotype × environment interactions and their partitions were significant for grain yield and endosperm hardness. Pool 24 QPM was the highest‐yielding parent (6.48 Mg ha‐1) and Pool 24 QPM × Population 63 was the highest‐yielding croos (6.56 Mg ha‐1) among hard endosperm parents. PR 7737, a soft endosperm opaque‐2, was low yielding as a parent, but performed better in crosses with other hard endosperm parents and showed high heterosis. High‐parent heterosis for grain yield was generally low in all crosses except those involving PR 7737 as a parent. Endosperm hardness ratings were intermediate relative to the parents, suggesting polygenic control. Crosses among white endosperm parents generally performed better than crosses among yellow endosperm parents. Crosses among dents and dent × flints yielded higher than flint crosses, but flint parents and their crosses had a superior endosperm modification. compared with dents. Populations 62, 63, and PR 7737 showed significant positive GCA effects for grain yield; Pool 23 QPM, Pool 25 QPM, and Populations 62, 64, and 65 had significant negative GCA effects for endosperm hardness. Populations 62 and 63 among white endosperm materials and Population 65 among yellow endosperm could be used for initiating hybrid development work. The broad genetic base provides opportunities for developing intrapopulation interline hybrids.

Soil acidity reduces maize (Zea mays L.) yields on about eight million hectares in developing countries. We report on response to selection for tolerance to soil acidity, using an altered version of modified ear‐to‐row (MER) and full‐sib (FS) selection. In the MER selection, 120 half‐sib (HS) families were evaluated under 45 and 80% AI saturation. One to three ears from each of the best 30% of the families were selected each cycle. After 16 cycles of MER selection, FS selection was initiated. Two hundred and fifty FS families were evaluated at five to six acid (ASEs) and one normal soil environments (NSE) the best 25% were selected each cycle. Cycles 2, 4, 6, 8, 10, 12, and 14 of MER and 0, 1, and 2 of FS selection were evaluated in three to nine replications at six ASEs and five NSEs during 1990–1991. Across the 11 environments, gain from selection in yield averaged 40 kg ha−1 cycle−1 (1.49**%) with MER and 250 kg ha−1 cycle−1v (8.10*%) with FS selection. Across the six ASEs, yield improvements of 40 kg ha−1 cycle−1 (1.99**%) with MER and 310 kg ha−1 cycle−1 (13.96**%) with FS selection were obtained. Yield also improved across the five NSEs by 50 kg ha−1 cycle−1 (1.10**%) with MER and 150 kg ha−1 cycle−1 (3.31%) with FS selection. Results indicate that tolerance to soil acidity can be improved with recurrent selection and that the progress will be higher with a system more efficient at reducing the experimental error and genotype ✕ environment interaction.