No abstract

Downy mildew [Pseudoperonospora cubensis (Berk. & Curt.) Rostov] is an important disease in most cucumber (Cucumis sativus L.) production areas of the world. Resistant cullivars are available, but higher levels of resistance are needed if yield losses are to be avoided. The objective of this experiment was to evaluate all available plant introduction accessions (from the U.S. National Plant Germplasm System), cultivars, and breeding lines (hereafter collectively referred to as cultigens) of cucumber for downy mildew resistance under field conditions in North Carolina. All available cultigens were tested in four blocks (2 yr and two replications) under natural field epidemics of the disease. Mean ratings for downy mildew leaf damage ranged from 1.3 to 9.0 on a 0 to 9 scale. The most resistant nine cultigens originated from the USA, and were primarily adapted cultivars or breeding lines. The most resistant cultigens, for which multiple‐year data were available, were Gy 4, ‘Clinton’, PI 234517, ‘Poinsett 76’, Gy 5, ‘Addis’, M 21, M 27, and ‘Galaxy’. The most susceptible cultigens for which multiple year data were available, were P1288995, P1176952, PI 178886, and P1211985. We classified 17 cultigens as highly resistant (1.3–3.0), 87 as moderately resistant (3.3–5.0), 311 as moderately susceptible (5.3–7.0), and 248 as highly susceptible (7.3–9.0) for the cultigens with multiple‐year data. No plant introduction accessions were found to be more resistant than the most resistant elite cultivars and breeding lines tested.

total area and production of slicing cucumber for 1997 was 9760 ha with a production of 202 514 Mg (USDA, Yield of cucumber (Cucumis sativus L.) in the United States has1998). Cucumber is the second most important vegetanot been significantly increased during the last two decades. Our objective was to evaluate the USDA cucumber germplasm collection ble crop in North Carolina, with a production area of for fruit yield and quality. All cucumber plant introduction accessions ≈9717 ha (USDA, 1997). Nationally, North Carolina from the USDA National Plant Germplasm System collection plus was ranked third in pickling cucumber production after check cultivars and breeding lines (hereafter collectively referred to Michigan and Florida, and fifth in slicing cucumber proas cultigens) were evaluated for early, total, and marketable yield duction after Georgia, Florida, Michigan, and California (number and weight), fruit quality rating, and days to harvest in during 1997 (USDA, 1997). small plots harvested once. All plants were treated with ethephonBreeding for yield in cucumber has been one of the (2-chloroethyl phosphonic acid) to make them gynoecious. Highly important objectives of many cucumber breeding prosignificant differences were observed among cultigens for all traits grams since the 1900s (Wehner, 1989). Yield of pickling evaluated in the study. Pickling type cultigens with the highest yield (fruit weight) were PI 209065, PI 326598, PI 137848, PI 285610, and cucumber has been improved by breeding for disease PI 264666. Slicing type cultigens with the highest yield were PI 234517, resistance (Peterson, 1975), as well as through the use PI 118279, PI 304085, and PI 512614. Beit Alpha type cultigens with of improved cultural practices (Cargill et al., 1975). Inthe highest yield were PI 167050, PI 163213, PI 532519, PI 211978, creased yield of cucumber cultivars has been due also to PI 357864, PI 183231, and PI 211117. Trellis type cultigens with the the improvement of qualitative traits such as gynoecious highest yield were PI 264228, PI 478366, PI 390262, PI 532524, PI sex expression, improved fruit color (improved percent-390267, and PI 532520. The USDA collection also exhibited a wide age marketable fruit), and direct yield improvement range in diversity for marketable fruit number, fruit weight, percent-(Wehner, 1989). age of culled fruit at harvest, fruit quality, and days to harvest. High The average yield of pickling cucumber in the United yielding cultigens identified in the study could be used to develop high yielding cultivars. Dep. Hortic. Sci., North Carolina State Univ., Raleigh, NC 27695-7609. The research was funded in part by the North Carolina Agricultural Once-over harvest trials having three replications Research Service (NCARS).

Gummy stem blight (Didymella blight), caused by Didymella bryoniae (Auersw.) Rehm and its anamorph Phoma cucurbitacearum (Fr.:Fr.) Sacc., is the second most important disease of cucumber (Cucumis sativus L.) in North Carolina after root knot nematodes Meloidogyne sp. Both Didymella blight and Phoma blight, caused by Phoma exigua Desm., have similar symptoms and control practices, and are generally referred to as gummy stem blight. In order to determine whether resistance existed to North Carolina isolates of D. bryoniae, 851 cultigens [cultivars, breeding lines, and plant introduction (PI) lines] were evaluated in the field. Plants were inoculated with one selected isolate (highly pathogenic in preliminary greenhouse tests) at the vine tip-over stage. They were rated for foliage lesion size and number on a 0 to 9 visual scale (0 = no disease, 9 = plant killed) and average ratings for 10 plants per plot were analyzed. The ratings ranged from 2.0 (highly resistant) to 8.5 (highly susceptible) with a mean of 6.2. The most resistant breeding lines and PI accessions were PI 200815, PI 390243, `LJ 90430', PI 279469, and PI 432855. The most resistant cultivars were `Homegreen #2', `Little John', `Transamerica', and `Poinsett 76'. The most susceptible cultigens in the study were PI 288238, PI 357843, PI 357865, and PI 167134. Two popular cultivars in North Carolina, `Calypso' and `Dasher II', were moderately resistant.

Gene linkage was investigated in 11 families using 18 genes in cucumber (Cucumis sativus L.). The genes studied were B (black spine), B-3 (Black spine-3), B-4 (Black spine-4), bi (bitterfree cotyledons), Bt (bitter fruit), Bt-2 (bitter fruit-2), D (dull fruit skin), df (delayed flowering), de (determinate habit), F (female sex expression), gl (glabrous foliage), lh (long hypocotyl), ns (numerous spines), pm-h [powdery mildew (Sphaerotheca fuliginea Schlecht.:Fr.) resistance expressed on the hypocotyl], ss (small spines), Tu (tuberculate fruit), u (uniform immature fruit color), and w (white immature fruit color). A major objective of this study was to measure linkages of genes for fruit bitterness (Bt and Bt-2), and spine color (B-3 and B-4) relative to previously studied loci: B, bi, D, de, df, F, gl, lh, ns, pm-h, ss, Tu, u, and w. The F2 progeny of LJ 90430 × PI 173889 segregated 13 bitter fruit: 3 nonbitter fruit, indicating that different genes are controlling fruit bitterness in these lines. Bt-2 is proposed as the gene controlling bitterness of fruit in LJ 90430. It is a separate locus from Bt, that causes bitter fruit in PI 173889. Several new gene linkages were found: bi—Bt, (Bt-2)—de, D—(Bt-2), D—ns, gl—F, ss—(Bt-2), Tu—(Bt-2), and u—(Bt-2). The Bt gene appears to be linked to bi and may be located on linkage group I. Bt-2 appears to be linked with several genes that could connect linkage groups I and IV. Bt-2 was linked to u, Tu, D, and ss, that are all on linkage group IV. Bt-2 was also found to be linked loosely to de, that is on linkage group I. No linkages were found between B-3 and B-4 and the genes evaluated in this study. Weak linkages (>25 cM) between several gene combinations [(Bt-2)-de, de—ns, de—ss, de—Tu, de—u, ns—F, and ss—F] provided more evidence that linkage group I and IV may be linked. Due to the weak linkages, more information needs to be obtained using larger populations and more markers to confirm these findings.