Powdery and Downy Mildew Resistance in Cucurbita moschata Accessions

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

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, University of Puerto Rico and was supported in part by the U.S. Dept. of Agriculture under CSRS Special Grant 88-34135-4661 managed by the Caribbean Basin Advisory Group (CBAG).