Alternaria alternata (Fr.) Keissl., (1912) f. sp. cucurbitae Vakal., (1990)

Alternaria leaf spot

- classification : Fungi, Ascomycota, Dothideomycetes, Pleosporomycetidae, Pleosporales, Pleosporaceae
- teleomorph: Lewia
- English name: Alternaria leaf spot

This early blight is caused by a specialized form of the fungus Alternaria alternata which appears to specifically attack a few Cucurbits. This disease has sometimes been devastating in several countries around the world. For example, attacks on cucumber, melon and watermelon are reported in greenhouses in Hungary in autumn 2001 and spring 2002. Severe losses are reported on cucumber, and to a lesser degree on melon, in 1979-80 in Crete . In the United States (in Maryland) in July 2006, the leaf area affected by the lesions caused by this early blight fluctuated between 3 and 20% depending on the melon cultivars.

This disease is also reported on bitter ( gourd Momordica charantia ), loofah ( Luffa cylindrica [ L. aegyptiaca ]) in Taiwan. It is particularly serious on cucumber, causing severe losses in most crops in Iceland.


Main symptoms

Symptoms caused by A. alternata f. sp. cucurbitae seem to occur mainly on the cotyledons of seedlings and on the leaves located on the central and upper parts of plants. Spots which rapidly become necrotic appear on the lamina, particularly at the periphery of the latter. These are more or less brown, darker at the periphery, surrounded by a chlorotic halo, and sometimes split at the end of the evolution. These spots can expand and coalesce to form circular to irregular patches, sometimes up to more than 5 cm in diameter. Severely infected leaves turn yellow and die prematurely.
Let us add that particularly diseased plants have fewer and smaller fruits.

A. alternata f. sp. cucurbitae sporulates rather frequently on damaged tissue in the form of a more or less dense black-brown conidial down. In addition, like other Alternaria , it is a common and opportunistic invader of senescent and injured tissue which is also frequently found on more or less damaged ripe fruits.


Biology, epidemiology

• Conservation, sources d'inoculum

A. alternata f. sp. cucurbitae certainly has saprophytic potentialities that allow it to be easily maintained on and in the soil, and on plant debris of all kinds (leaflets, stems, senescent floral pieces ), from Cucurbitaceae crops, but probably many alternative hosts , cultivated or not. It can also pollute the seeds which will ensure its preservation and dissemination.

• Penetration, invasion

The conidia present on the leaves germinate and emit one to several mycelial filaments which can penetrate directly through the cuticle or via various wounds present on the different organs of Cucurbitaceae. The mycelium invades the tissues and lesions can already be observed after 4 to 5 days of incubation if the climatic conditions are favorable for the development of the fungus.

• Sporulation and dissemination

As we have already mentioned previously, this Alternaria sporulates easily on altered tissues, it produces conidiophores carrying conidia which will be dispersed by wind and air currents, but also by splashing water. These conidia will ensure secondary contaminations.

It should be remembered that the seeds can also be the source of the spread of early blight.

• Conditions favorable to its development

This fungus particularly appreciates humidity. Rainy periods and the presence of water on the plant cover favor its development. The optimum temperature for the germination of its conidia and for the growth of its mycelium is close to 26 ° C. Conidia are likely to germinate in water within 2 hours and at temperatures between 10 and 37 ° C.
Plants that are not very vigorous and loaded with fruit are more sensitive.

Protection methods

We advise you to refer to the "Protection methods" paragraph of the sheet A. cucumerina because the control methods to be implemented to control A. alternata f. sp. cucurbitae are identical with a few exceptions.

Let's add that a few varieties of cucumber would be resistant to this fungus. In addition, in vitro studies have shown the antagonistic of potential Streptomyces lydicus against A. alternata f. sp. cucurbitae .


* Chemical control : As the number of pesticides available for a given use is constantly changing, we advise you to always confirm your choice by consulting the e-phy site of the Ministry of Agriculture and Fisheries which is an online catalog of plant protection products and their uses, fertilizers and growing media approved in France. This also applies to all biological products based on microorganisms or natural substances.