Cucurbit Genetics Cooperative Report 7:38-40 (article 17) 1984

Evaluating Downy Mildew Resistance in Cucumis melo L.

Cohen, Y., H. Eyal and A. Cohen

Department of Life Sciences, Bar-Ilan University, Israel

C.E. Thomas

U.S. Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory, Charleston, SC 29407

Downy mildew incited by Pseudoperonospora cubensis (Berk. & Curt.) Rostow. is an important disease on cantaloup (C. melo) in many production areas of both the U.S.A. and Israel. A major phase of our research has involved the evaluation of C. melo germplasm for resistance against this disease. As better sources of resistance have been identified, modifications have been made in our screening procedures to accommodate them. From 1977-82, the yellow vs. brown lesion reaction served as the basic criterion for evaluating young plants inoculated at the 2-leaf stage. This reaction was first described by Barnes and Epps (1) in cucumber, and was elaborated by Thomas in cantaloup (2). The following 1-9 scale has been used by Thomas and others to evaluate gradations of the yellow vs. brown lesion reaction observed:

1. entire leaf necrotic
2. coalesced yellow lesions, general sporulation
3. yellow lesions, moderate to abundant sporulation
4. brown lesions with yellow halo, moderate sporulation
5. large brown lesions, light to moderate sporulation
6. medium-sized brown lesions, light sporulation
7. small brown lesions, trace to very light sporulation
8. pin-prick brown lesions, no sporulation
9. entire leaf healthy, no evidence of infection

The main problem with this rating system was that leaf 1 and leaf 2 did not always react the same. Except in those cases where they did react the same, leaf 1 always exhibited a more susceptible reaction than did leaf 2. Therefore, the one digit rating assigned represented the most severe reaction present to avoid overestimating resistance.

In field plots a 1-9 scale was also used, but this scale rated gradations in amount of observed infection and subsequent disease damage as expressed by numbers of lesions and leaf loss. Since it did not rate lesion (reaction) type, correlations between artificial inoculations in the 2-leaf stage and reactions of older plants in field plots were difficult. Partly because of this difficulty, Thomas (3) used percent leaf loss to more critically describe the reaction of selected plant introductions and cultivars in field plots. This technique was very effective, but labor intensive.

More recently, due to the recognition of a different reaction (lesion) type as an expression of resistance and incorporation of this resistance into our breeding programs, a new protocol has been devised. This reaction type (RT) is characterized by 1 mm, circular, water-soaked, yellow lesions with no apparent sporulation. This protocol, which we refer to as a double rating technique, accommodates the newly recognized RT and better facilitates correlations between reactions of plants inoculated at the 2-leaf stage and older plants in the field.

Recognition of this resistant RT is difficult unless the following inoculation and post-inoculation procedures and conditions are followed. Plants are grown in the greenhouse (day:night temperature 20±2 C: 27±3 C) to the 2-true leaf stage, one plant per pot. The adaxial leaf surfaces are inoculated at about 1800 hr with a 1.0 x 10⁴ sporangial suspension with a hand sprayer. Inoculated plants are placed overnight in a high humidity tent and then transferred to the greenhouse bench until seven days after inoculation, when they are returned to the high humidity tent for 20 hr to induce fungal sporulation. Disease evaluations are made separately for leaf 1 and leaf 2 on each plant using the following index of RT's:

1. 10-15 mm, chlorotic, irregular, heavily-sporulating lesions
2. type '1' lesions, above, mixed with type '3' lesions, below
3. 3-4 mm, irregular to circular, water-soaked, chlorotic, sparsely sporulating lesions
4. 1 mm, circular, water-soaked, yellow lesions with no apparent sporulation

After this evaluation, plants are transplanted to the field (mid- April through mid-September) and inoculated with P. cubensis as described above when they reach the 10-leaf stage. Eight to 10 days after this field inoculation, separate evaluations are made for leaves 3-6 and leaves 7-10 using the reaction type index described above.

The above evaluation procedures result in a two digit number to describe the reaction of inoculated plants at either the 2-leaf or 10-leaf stage. The first digit (1-4) expresses the reaction of leaf 1 (or of leaves 3-6) and the second digit expresses that of leaf 2 (or of leaves 7-10) in the greenhouse (or in the field). Observed RT's range from 11, highly susceptible, to 44, highly resistant.

Using this protocol we have observed that on 2-leaf plants in the greenhouse, RT4 on leaf 1 was always associated with RT4 on leaf 2; whereas, RT4 on leaf 2 may have been associated with RT's 2 or 3 on leaf 1. Similarly in the field with 10-leaf plants, RT4 in lower leaves was always associated with RT4 in upper leaves; whereas RT4 in upper leaves may have been associated with RT's 2 or 3 in lower leaves.

F₂ populations of crosses made between a highly resistant breeding line (RT 44 in both greenhouse and field) and the cultivars 'Hemed', 'Ein-Dor' and 'Ananas-Yokneam' (RT's 11 and 12) segregated in greenhouse tests to RT's 11, 12, 13, 23, 24, 33, 34, and 44, with 3.2-12% of the populations showing RT 44. Most plants reacting with a RT 44 in the greenhouse (2-leaf) tests retained a RT 44 in the field until maturity, while none of the plants reacting with RT's 11-33 did so. Occasionally, plants reacting with RT 34 in the greenhouse exhibited a higher degree of resistance (RT 44) in the field.

The double rating technique described above facilitates the selection of highly resistant (RT's 34 and 44) plants both in the greenhouse and in the field. If RT4 is present in the germplasm and its selection is the objective of the evaluation, we recommend use of this technique to identify plants exhibiting RT 34 and 44. If RT4 is not present, we suggest use of the 1-9 rating scale to assess the yellow vs. brown lesion reaction.

Literature Cited

1. Barnes, W.C. and W.M. Epps. 1954. An unreported type of resistance to cucumber downy mildew. Plant Dis. Reptr. 38:620.
2. Thomas, C.E. 1978. Reaction of susceptible and resistant cantaloups to Pseudoperonospora cubensis. Plant Dis. Reptr. 62:221-222.
3. Thomas, C.E. 1982. Resistance to downy mildew in Cucumis melo plant introductions and American cultivars. Plant Dis. Reptr. 66:500-502.

This research was supported by Grant No. US-287- 81 from BARD--The United States-Israel Binational Agricultural Research & Development Fund.