Downy Mildew Resistance of the Cucumber Germplasm Collection in North Carolina Field Tests

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.

Evaluation of selected Cucumis sativus accessions for resistance to Pseudoperonospora cubensis in Egypt

“…low diversity), downy mildew resistance has been identified in the USA. PIs germplasm collection in recent decades (Staub et al 1989;Wehner & Shetty 1997;Klosinska et al 2010;Call et al 2012). In this study PIs with higher levels of resistance have been identified in several accessions.…”

“…Ames 2353, Ames 2354 and PI 234517 were major sources of downy mildew resistance in the United States (Call et al 2012), but were susceptible in this study with mean ratings of 7 or 8. Furthermore, PI 234517, which was determined to be highly resistant by Wehner and Shetty (1997), was susceptible in our test. Therefore, we recommend that cucumber cultivars developed in the U.S. be tested for resistance to P. cubensis using the Egyptian race(s) of the pathogen before release.…”

“…Seeds of 133 C. sativus PIs were obtained from the USDA, ARS North Central Regional Plant Introduction Station (NCRPIS) in Ames, Iowa (Table 1) and included accessions from 19 countries. Most accessions were selected based on previous studies reporting moderate to high downy mildew resistance (Wehner & Shetty 1997;Call et al 2012). Additional information on C. sativus PIs used in this study is available at http://www.ars-grin.…”

“…Plots were rated for symptoms five weeks after transplanting on a 0 to 9 scale based on the percentage of symptomatic leaf area (0 = 0%, 1 =1-5%, 2 = 6-10%, 3 = 11-20%, 4 = 21-30%, 5 = 31-50%, 6 = 51-65%, 7 = 66-80%, 8 = 81-99%, and 9 = 100%). The classification of accessions into resistant and susceptible groups is somewhat subjective but in keeping with previous studies (Wehner & Shetty 1997;Call et al 2012) accessions were grouped by mean downy mildew rating as follows: ≤ 2.0 (highly resistant); 2.1 to 4.0 (moderately resistant); 4.1 to 6.0 (intermediate); 6.1 to 7.0 (moderately susceptible); 7.1 to 9.0 (highly susceptible).…”

“…Perhaps the most comprehensive study was conducted over multiple years by Wehner and Shetty (1997) in which the whole U.S. germplasm cucumber collection was tested doi: 10.17221/12/2015-CJGPB for resistance to downy mildew in North Carolina. The most-resistant cultigens were Gy 4, Clinton, Plant Introduction (PI) 234517, Poinsett 76, Gy 5, Addis, M 21, M 27, and Galaxy.…”