Cucurbit Genetics Cooperative Report 6:10-12 (article 5) 1983
S. F. Jenkins, Jr. and T. C. Wehner
North Carolina State University, Raleigh, NC 27650
Several pathogenic microorganisms cause injury to the foliage of pickling and slicing cucumbers (Table 1). The pathogens may cause injury to all above-ground parts including the leaves, stems, petioles, peduncles and fruit. Cucumber cultivars have been developed with resistance to many of the foliar diseases. In addition, effective chemical controls have been developed for most of the fungal diseases. As the amount of cooperative research among programs increases, it is important that we be able to measure the incidence of disease accurately in order to evaluate genetic resistance and cultural methods of control.
Table 1. Principle diseases and causal agents of foliar diseases of cucumber.
Disease |
Causal Agent |
| Anthracnose | Colletotrichum lagenarium |
| Angular leaf spot | Pseudomonas lachrymans |
| Downy mildew | Pseudoperonospora cubensis |
| Powdery mildew | Erysiphe cichoracearum |
| Scab | Cladosporium cucumerinum |
| Cucumber mosaic | Cucumber mosaic virus |
| Target spot | Corynespora cassiicola |
| Gummy stem blight | Didymella bryoniae |
| Alternaria leaf spot | Alternaria cucumerinum |
| Air pollution | Ozone, sulfur dioxide, etc. |
Several systems for rating foliar disease on crop plants have been developed. The systems vary from simple (e.g. + or -, diseased or healthy) to complex (e.g. 1 to 100, percent diseased). Horsfall and Barratt (2) developed a scoring system which is based on the Weber-Fechner Law, that the human eye distinguishes changes on the leaf according to the logarithm of the light intensity. That system (Table 2) has as its midpoint 50 percent foliar disease. One grade above and below 50 percent, the percent disease changes by a factor of two. The system has 12 grades in which the percent disease ranges from 0 to 100. Redman et al. (3) developed tables which converted the Horsfall-Barratt system to various percentage estimates of disease. Their scale ranges from 0 to 11 while the original Horsfall-Barratt scale ranges from 1 to 12 (Table 2).
Table 2. Comparison of disease rating scales of Horsfall-Barratt, Redman et al. and Jenkins-Wehner.
Horsfall-Barratt score |
Redman et al. score |
% disease |
Mean % conversionz |
Jenkins-Wehner
|
Jenkins-Wehner
|
Jenkins-Wehner
|
| 1 | 0 | 0 | 0.00 | 0 | 0 | 0.1 |
| 2 | 1 | 0–3 | 2.34 | 1 | 0–3 | 1.5 |
| 3 | 2 | 3–6 | 4.68 | 2 | 3–6 | 4.5 |
| 4 | 3 | 6–12 | 9.37 | 3 | 6–12 | 9.0 |
| 5 | 4 | 12–25 | 18.75 | 4 | 12–25 | 18.5 |
| 6 | 5 | 25–50 | 37.50 | 5 | 25–50 | 37.5 |
| 7 | 6 | 50–75 | 62.50 | 6 | 50–75 | 62.5 |
| 8 | 7 | 75–87 | 81.25 | 7 | 75–87 | 81.5 |
| 9 | 8 | 87–94 | 90.63 | 8 | 87–100 | 93.5 |
| 10 | 9 | 94–97 | 95.31 | 9 | 100 | 99.9 |
| 11 | 10 | 97–100 | 97.66 | – | – | – |
| 12 | 11 | 100 | 100.00 | – | – | – |
z Conversion factor is used to convert the score to percent foliage damage.
y Verbal description of scores 0 to 9 of Jenkins – Wehner:
0 = no disease; immune
1 = few small leaf lesions; highly resistant
2 = few lesions on few leaves with no stem lesions; resistant
3 = few lesions on few leaves or with superficial stem lesions; moderately resistant
4 = few well-formed leaf lesions or superficial stem lesions; intermediate
5 = few well-formed leaf lesions or enlarging stem lesions; intermediate
6 = many large leaf lesions or deep stem lesions with abundant sporulation, or plant more than 50% defoliated; susceptible
7 = many large coalescing leaf or stem lesions, over 75% of plant area affected or defoliated; susceptible
8 = plants largely defoliated, leaves or stems with abundant sporulating lesions; highly susceptible
9 = plants dead; highly susceptible
Our proposed scale for evaluating cucumber foliar disease is a modification of a scale using the scores 1 through 5 developed by Goode (1) for evaluating anthracnose on cucurbits. This system is as follows: 1 = highly resistant or immune, no lesions; 2 = resistant, with a few small leaf lesions; 3 = intermediate, with a few well-formed leaf lesions or superficial stem lesions; 4 = susceptible, with many large leaf lesions or deep stem lesions and usually with abundant sporulation; 5 = highly susceptible, plants dead.
We expanded that scale to include more intermediate categories (Table 2). The scale we developed ranges from 0 to 9 and has the advantage of using only one column on the data sheet instead of two as the Horsfall-Barratt scale does. Our scale is essentially a hybrid Of Goode’s scale (1) and the Horsfall-Barratt scale (2). We have condensed Horsfall-Barratt grades 9 (87 to 94%), 10 (94 to 97%) and 11 (97 to 100%) into our grade 8 (87 to 100%) because we have observed that once a cucumber plant reaches that stage of disease (87% foliar damage) it is no longer productive. On the lower end of the scale, it is much easier to detect differences and those differences are significant in predicting disease development. We propose that cucumber foliar diseases be scored in a standard fashion using the 0 to 9 scale.
Literature Cited
- Goode, M. J. 1958. Physiological specialization in Colletotrichum lagenarium. Phytopathology 48:79–83.
- Horsfall, J. G. and R. W. Barratt. 1945. An improved grading system for measuring plant diseases. Phytopathology 35:655 (Abstr.).
- Redman, G. E., E. P. King, and I. F. Brown, Jr. Elanco Conversion Tables for Barratt-Horsfall Rating Numbers. Elanco Products Co. Indianapolis, Indiana.