Cucurbit Genetics Cooperative Report 12:1-4 (article 1) 1989

Alternaria alternata f. sp. cucurbitae on Cucumber and Other Cucurbits

Demetrios John Vakalounakis

Plant Protection Institute, Heraklio, Crete, Greece

During the 1979 to 1980 crop season, a severe leaf spot disease of cucumber (Cucumis sativus L.) was noticed on greenhouse crops grown in some plastic houses in the Sitia Area, Lasithi, Crete, Greece, along the coastal strip between Koutsouras and Goudouras (10). Since then, it has spread to most of the cucumber growing areas in Crete, causing severe losses.

Symptoms appear in late autumn, mainly on the leaves of the middle and upper part of the plants. Necrotic flecks, surrounded by a chlorotic halo, appear on the leaf, and these enlarge to spots which may coalesce to form lesions up to 5 cm or more in diameter. The lesions appear circular in shape and bear black-brown fructifications of the pathogen. Severely-infected leaves become yellow, senescent, and die. No other part of the plant is affected. During the winter, when relative humidity in the plastic houses is high and plant vigor is reduced due to fruit bearing and unfavorable climatic conditions (reduced illumination and average air temperature lower than 15°C), infection progresses rapidly throughout the crop resulting in severe damage within a few days (11).

A long-chained Alternaria spp. with small spores was always observed on the old lesions of infected cucumber leaves. The same fungus was consistently obtained from samples taken from different plastic houses when pieces of infected tissue or spores from the spots were plated out on Petri dishes containing potato dextrose agar (PDA). The cultures of the fungus on PDA at 25°C under "daylight" fluorescent lamps have a dirty white color at the beginning, while later the center becomes gray. In a few days, the entire surface is covered with an abundance of spores. The spores on infected leaves or on cultures on PDA are produced in long chains on short conidiophores. They are brown but, when many of them have been produced on PDA, they look black with a velvety appearance. The dimensions of the spores either in vivo or in vitro (Table 1) agree fairly well with published descriptions of Alternaria alternata (Fr.) Keissler (8) and its synonym Alternaria tenuis Auct. (6, 7). The pathogen of the present disease is also similar to A. alternata f. sp. lycopersici which causes a stem canker of tomato (2). Alternaria infections similar to those described in this paper are very common on cucumber but are caused by Alternaria cucumerina (Ellis & Everh.) Elliott (synonym Alternaria brassicae f. nigrescens Pegl.) (1, 5) or Alternaria pluriseptata (Karst. & Har.) Jorstad [synonyms Alternaria cucurbitae Let. & Roum., Stemphylium ilicis Tengwall, Ulocladium cucurbitae (Let. & Roum.) Simmons, Ulocladuim atrum Press] (3, 9). However, both these fungi are morphologically distinct (1, 4, 5, 6) and are readily distinguishable from Alternaria alternata (11).

Of 62 cultivated and weedy species in 16 botanical families artificially inoculated and naturally infected in greenhouse experiments, 27 species belonging to the Cucurbitaceae were found to be susceptible to the pathogen (Table 2).

Table 1. Morphological characteristics of conidia of Alternaria alternata f. sp. cucurbitae from cucumber leaf spots in comparison with those of published descriptions of A. alternata, A. cucumerina and A. pluriseptata.

Spore measurements (µm)	Body length	Body width	Beak length	Spores (% with beaks)	Total length	No. of septa	Spores/chain
A. alternata
Cucumber leaf spot	3804±12.0z (15-68) y	14.6±3.3 (9-24)	6.3±4.4 (1-21)	75	42.2±14.3 (15-73)	2-8	4-5
PDA	20.1±4.8 (12-29)	9.7±2.0 (6-14)	5.1±2.3 (3-12)	57	25.3±4.5 (17-34)	2-5	8
A. alternata (Simmons, 1967)
Neotype specimen	30.9 (18-47)	12.6 (7-18)	up to 25	-	-	3-8	-
A. tenuis
Medium	25.7 (7-70)	11.2 (6-20)	5 (1-58)	80	7-72	1-6	8
A. alternata f. sp. lycopersici
Tomato	32.3±2.8 (18-50)	12.4±2.8 (7-18)	6.8±6.0 (2-20)	72	18-68	1-5	3-4
A. cucumerina (Jackson, 1958)
Host	57-87	18-21	106-135	-	-	5-9	-
A. pluriseptata (Hervert et al., 1980)
Host	19-66	8-16	-	-	-	2-9	-
Ulocladium atrum (Simmons, 1976)
Medium	18.6 16.5-19.8	16.0 13.2-18.7	no beaks	-	-	1-3	1; sometimes 2

z Mean ± standard deviation.
y Numbers in parentheses indicate extreme values.

Table 2. Susceptibility of cucurbitaceous species and some belonging to other families to infection by Alternaria alternata f. sp. cucurbitae.

Species	Disease severity
CUCURBITACEAE
Benincasa hispida ( Thumb.) Cogn.	+++
Citrullus lanatus ( Thumb.) Mansf.	++++
Cucumis africanus L. f.	+
Cucumis anguria L.	+++
Cucumis dipsaceus Ehrenb.	+
Cucumis ficifolius A. Rich	++
Cucumis hardwickii Royle	++++
Cucumis longipes Hook.	++
Cucumis melo L.	++++
Cucumis pustulatus	++++
Cucumis sativus L.	+++++
Cucurbita ficifolia B.	+
Cucurbita foetidissima Kunth.	+
Cucurbita lundelliana Bailey	++
Cucurbita maxima Duch.	+
Cucurbita mixta Pang.	+++
Cucurbita moschata (duch.) Duch. ex Poir.	+++
Cucurbita palmeta Wats.	+++
Cucurbita pepo L.	+++
Cucurbita sororia	+++
Cucurbita texana A. Gray	+
Ecballium elaterium (L.) A. Rich	+
Lagenarial leucantha Rusby var. clavata Makino	+++++
Lagenaria siceraria (Mol.) Standl. ssp. asiatica (Kob). Heiser	++++
Lagenaria vulgaris Ser.	+++++
Luffa cylindrica Roem.	++
Momordica charantia L.	+++
SOLANACEAE
Capsicum annuum L.	-
Lycopersicon esculentum Mill.	-
Nicotiana tabacum L.	-
Nicotiana glutinosa L.	-
Solanum melongena L.	-
UMBELIFERAE
Apium graveolens L.	-
Daucus carota L.	-
URTICACEAE
Urtica urens L.	-
AMARANTHACEAE
Amaranthus retroflexus L.	-
Gomphrena globosa L.	-
CHENOPODIACEAE
Beta Vulgaris L.	-
Chenopodium album L.	-
Spinacia oleracea L.	-
COMPOSITAE
Aster squamatus ( Spr.) Hier.	-
Cichorium endivia L.	-
Chichorium intybus L.	-
Erigeron canadensis L.	-
Lactuca sativa L.	-
Lactuca serriola L.	-
Sonchus asper (L.) Hill.	-
CONVOLVULACEAE
Convolvulus arvensis L.	-
CRUCIFERAE
Brassica oleracea L. var. capitata	-
Raphanus sativus L.	-
CYPERACEAE
Cyperus longus L.	-
var. badius Desf.	-
GERANIACEAE
Erodium malacoides Willd.	-
GRAMINAE
Setaria viridis P.B.	-
LEGUMINOSAE
Glycyrriza glabra L.	-
Medicago polymorpha L.	-
Melilotus indica All.	-
Phaseolus vulgaris L.	-
Vicia faba L.	-
LILIACEAE
Allium cepa L.	-
OXALIDACEAE
Oxalis corniculata L.	-
ROSACEAE
Fragaria vesca L.	-

- no disease; + to +++++ increasing amount of disease.

Literature Cited

1. Ellis, M.B., and P. Holliday. 1970. Alternaria cucumerina. C.M.I. Descriptions of pathogenic Fungi and Bacteria. No. 244.
2. Grogan, R.G., K.A. Kimble, and I. Misaghi. 1975. A stem canker disease of tomato caused by Alternaria alternata f. sp. lycoperisici. Phytopathology 65: 880-886.
3. Hervert, V., L. Marvanova, and V. Kazda. 1980. Alternaria pluriseptata on cucumbers and remarks to its classification. Ceska Mycologie 34: 13-20.
4. Jackson, K.R. 1958. Taxonomy and host range of Alternaria cucumerina. Phytopathology 48: 343-344.
5. Jackson, C.R., and G.F. Weber. 1959. Morphology and taxonomy of alternaria cucumerina. Mycologia 51: 401-408.
6. Groves, J.W., and A.J Skolko. 1944. Notes on seedborne fungi. II. Alternaria. Can. J. Res. 22:217-234.
7. Neergaard, P. 1945. Danish species of Alternaria and Stemphylium. Oxford University Press, London. 560 pp.
8. Simmons, E.G. 1967. Typification of Alternaria, Stemphylium, and Ulocladium. Mycologia 59: 67-92.
9. Simmons, E.G. 1982. Alternaria themes and variations (11-13). Mycotaxon 14: 44-57.
10. Vakalounakis, D.J. and N.E. Malathrakis. 1982. A cucumber disease caused by the fungus Alternaria alternata. 2nd Conf. Protected Veg. Flowers, Heraklion, Crete, Greece.
11. Vakalounakis, D.J. and N.E. Malathrakis. 1988. A cucumber disease caused by Alternaria alternata and its control. Z. Phytopathology 121: 325-336.