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Cucurbit Genetics Cooperative Report 16:73-74 (article 26) 1993

Powdery and Downy Mildew Resistance in Cucurbita moschata Accessions

Linda Wessel-Beaver

Dept. of Agronomy and Soils, College of Agricultural Sciences

University of Puerto Rico, Mayaguez, PR 00681

Powdery mildew is a disease found wherever cucurbit crops are grown. Two organisms are reported as the causal agents of this disease: Erysiphe cichoraceatum DC ex. Merat and Sphaerotheca fuliginea (Schlecht. ex. Fr. Poll. (3). In Puerto Rico E. cichoraceatum is the most prevalent pathogen (based on germ tube studies (2) although S. fuliginea is occasionally found (1). With the introduction of drip irrigation and plastic mulches in Puerto Rico, pumpkins are now grown on a commercial scale during the cool dry season. These conditions favor powdery mildew. Downy mildew (Psuedoperonospora cubensis) prefers somewhat warmer and wetter conditions than does powdery mildew (3). In the Caribbean and Central America where C. moschata is often grown with few or low-cost inputs, farmers traditionally plant in the warm rainy season because of lack of irrigtion equipment. In Puerto Rico downy mildew can limit production during the summer months or whenever unseasonable rains fall.

In this study all available accessions of C. moschata from the Southern Regional Plant Introduction Station, Experiment, Georgia were tested for powdery mildew resiistance in the field and greenhouse and for downy mildew resistance in the field at Isabela, PR (18 N latitude, elevation 131 m). For the field test accessions were direct-seeded on 5 December 1991 in plots consisting of single 3.9 m-wide rows with 4 plants spaced 1,2 m apart. Two replications (blocks) of each of the 343 accessions were planted. Eighteen of the 343 accessions did not germinate, 8 accessions had poor germination (fewer plants were tested and these accessions were not grown in the greenhouse) and 27 accessions were clearly not C. moschata or included mixtures of other Cucurbita spp. Powdery and downy mildew field ratings were taken on 23 January and 12 to 14 February, 1992. Conditions were excellent for the development of both diseases since a warm, rainy period (28 C day/24 C night) accompanied the planting, followed by cooler, dry weather (25 C day/22 C night). A 0 to 5 scale was used: 0 = no mildew; 1 = less than 1 lesion per leaf; 2 = 1 lesion per leaf; 3 = several sporulating lesions per leaf, some mildew on petioles or stems; 4 = many sporulating lesions on leaves, petioles and stems; 5 = most leaves, petioles and stems completely mildewed, leaves dessicated or dead. The greenhouse evaluation for powdery mildew consisted of two single-plant replications (blocks) of each accession. Block 1 was planted on 17 March and block 2 on 21 April 1992. Plants were inoculated by dusting with infected leaf tissue and rated as in the field.

Accessions having a mean powdery mildew rating of < 1.5 on the second field evaluationd ate are included in Table 1. Field ratings over all accessions ranged from0 to 5 with a mean rating of 2.7, an LSD of 1.06, and a CV of 19.8%. Almost half of the resistant accessions appeared to be something other than C. moschata. The resistant accessions came from very diverse origina. In the greenhouse most accessions were highly susceptible (Table 1). An exception was PI 438811 from Mexico.

Forty-five accessions showed field resistance (a mean rating of 0) to downy mildew (Table 2). Field ratings ove all accession ranged from 0 to 5 with a mean of 2.2, an LSD of 1.3 and a CV of 54.6%. Again, some of the resistant accessions appear to not be C. moschata. All resistant accessions are from Central America (mainly Mexico) with the exception of two PI from India. Most accessions collected in temperate regions were highly susceptible to powdery mildew.

A rather large amount of phenotypic variation was observed within many acessions. Correlation between field and greenhouse powdery mildew ratings was very low. Larger numbers of plants from accessions showing some resistance are currently being evaluated.

Table 1. Field and greenhouse powdery mildew ratings for C. moschata PI's with a meanfield rating of < 1.5.

PI Num.
Origin
Field
Greenhouse
PI Num.
Origin
Field
Greenhouse
193499
Ethiopia1
1.0
3.0
379295
Yugoslavia1
0.0
0.0
201254
Mexico1
1.5
---
414906
India1
1.0
5.0
234251
Japan
1.5
4.0
438811
Mexico
1.5
2.0
249565
Thailand
1.5
5.0
482490
Zimbabwe
1.0
4.0
298036
Australia1
0.0
0.0
282523
Zimbabwe1
1.5
2.5
357916
Yugoslavia
1.5
3.0
540906
Unknown
1.5
4.0
2693456
Costa Rica
1.5
3.5
----
----
---
---

1 Accession appears to be misclassified as C. moschata

Table 2. C. moschata accessions with a mean field rating of 0 for downy mildew.

PI Number
Origin
PI Number
Origin
PI Number
Origin
168547
Mexico
190185
Mexico1
196923
Mexico1
200736
El Salvador
201254
Mexico1
201471
Mexico
201473
Mexico
326184
Mexico1
281810
India
381815
India
438577
Guatemala
438578
Guatemala
438723
Mexico
438726
Mexico
438731
Mexico
438747
Mexico
438748
Mexico
438756
Mexico
438760
Mexico
438772
Mexico
438775
Mexico
438776
Mexico
438781
Mexico
438784
Mexico
438787
Mexico
438790
Mexico
438792
Mexico1
438794
Mexico
438824
Mexico
442248
Mexico
442249
Mexico
442250
Mexico
442251
Mexico
442253
Mexico
442256
Mexico
442257
Mexico
442258
Mexico
442272
Mexico
442274
Mexico
452276
Mexico
442281
Mexico
442284
Mexico
451836
Guatemala
451837
Guatemala
451845
Guatemala

1Accession appears to be misclassified as C. moschata.

Literature Cited

  1. Cienfuegos-Agreda, R.E. 1991. Metodos de inoculacion para cuantificar resistencia poligenica a anublo polvoriento Cucurbita moschata [(Duch) ex. Poir.]. M.S. Thesis, University of Puerto Rico, Mayaguez Campus, Mayaguez, PR. 62 pp.
  2. Hiiata, K. 1955. On the shape of germ tubes of Erysiphae (II). Bul. Faculty Agric Niigata Univ. 7:24-26 (In Japanese with English summary).
  3. Siterly, W.R. 1978. Powdery mildews of cucurbits, p. 359-379. In: D.M. Spencer (ed). The powdery mildews, Academic Press Inc., New York.

This research is a contribution from the Agricultural Experiment Station, UIniversioty of Puerto Rico and was supported in part by the U.S. Dept. of Agricultureunder CSRS Special Grant 88-34135-4661 managed by the Caribbean Basin Advisory Group (CBAG).

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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 15 December, 2009