Diaporthe melonis Beraha & M.J. O'Brien (1979)

Black rot and "purple stem"

- classification : Fungi, Ascomycota, Sordariomycetes, Sordariomycetidae, Diaporthales, Diaporthaceae
- synonymously, Phomopsis squash McKeen (1957)
- English name: black rot and purple stem


Diaporthe melonis soil is a fungus that has been reported from several production regions around the world, including India, Japan, the United States and Canada, primarily affecting melon , cucumber and watermelon. Its impact on these crops does not appear to be marked.

This fungus is not rife in France on Cucurbitaceae.


Main symptoms

Watery lesions, reddish to purple, initiate and extend to the lower part of the stem . Subsequently, they lighten and turn white with age. Gummy, amber exudates usually appear on the lesions. When the stem is surrounded by a canker, the leaves wither and wither and the plants wither.

D. melonis can be responsible for rots on fruits melon , sometimes starting from the stylar scar. In the case of immature melons, infections may remain latent and only manifest themselves after harvest, during storage and preservation.

Note that the lesions on the stem are generally more or less covered with black globular structures: pycnidia sometimes aligned in rows. This would not be the case on rotten fruit.


Biology, epidemiology

The information available on the biology and epidemiology of D. melonis is still limited.

• Conservation, source d'inoculum

The storage methods for D. melonis are not known. It must certainly be able to be preserved through its mycelium and these pycnidia like many other species of Phomopsis .

• Penetration, invasion
  

The infection of the organs is generally initiated on senescent tissues and in particular in places on plants where they are frequent: junctions of the stem and petioles, tendrils, internodes . Senescent floral parts constitute nutrient bases very favorable to the penetration of D. melonis .

Once in place, the mycelium invades the epidermal and cortical tissues, as well as the vascular bundles which are eventually destroyed.

• Sporulation and dissemination

This fungus produces numerous black globular structures of various shapes on lesions: pycnidia. Their size is about 0.5mm. These produce at the end of hyaline conidiophores and two types of conidia called alpha or beta depending on their shape. Alpha conidia are hyaline, unicellular, spindle-shaped to ellipsoidal, and biggutulate. Their average dimensions are 8.3 × 2.6 µm. Beta conidia are hyaline, threadlike, unicellular, and curved at one end. They measure on average 24.7 × 1.3 µm.

This pathogen appears to produce mainly alpha-type pycnidia and conidia.

These conidia are extruded from the pycnidia in the form of mucous clumps or cirrhia, and they are disseminated mainly by splashing water, tools or even workers during their work in crops, in particular if these are wet.

• Conditions favorable to its development

conditions favorable to the development of Little D. melonis is known about the . The fungus would be favored by the high humidity conditions.

Like many aerial fungi, it is particularly fond of humid environments and rainy periods Plants that are etiolated and / or harbor numerous senescent tissues are also probably more vulnerable.


Protection methods

As with the biology of D. melonis , little information is available concerning the protection methods to be implemented to control this fungus. However, it is advisable to combine all the measures and methods to eliminate it or limit its development.

• During cultivation

The first diseased plant (s) must be removed with care, in order to limit the spread of this fungus. To do this, they should be placed in a plastic bag to avoid any contact with other healthy surrounding plants when they are removed from the crop. They will be quickly destroyed thereafter. Once the disease is present in the crop, visitors should be avoided, and crops should be worked when the plants are dry.

This fungus appreciates humid climatic conditions, it is therefore essential to ventilate the greenhouses well in order to reduce the humidity and avoid water condensation on the plants.

No fungicides have been reported to be effective against D. melonis .

At the end of cultivation, the plant debris will be eliminated; they will in no case be piled up and kept near the plots, as they will subsequently constitute sources of inoculum. It will be better to destroy them by using quicklime or by burning them. Otherwise, the pile of debris will be covered with a plastic film to form a mechanical barrier.

• Next crop

It will be important to put in place prophylaxis measures. In cultivation under cover, in particular above ground, the entire farm must be disinfected in order to get rid of as many propagules as possible that could contaminate the new plants. This will require disinfecting the surface of the internal structures of the shelters. Bags, substrates, tools and other materials that may have been contaminated should be disposed of. If they are reused, they will need to be disinfected beforehand.

No variety has been identified as resistant.

It will be essential to use healthy plants which must be produced with a healthy substrate. In addition, avoid overwatering the plants, which will limit the risk of the fungus developing. It will be necessary to be wary of the sanitary quality of the water used for the preparation of the nutrient solution and / or the irrigation of the plants, especially if it comes from an irrigation canal, a watercourse, 'a basin that may have been contaminated. The pipes of the drip network will have to be cleaned or replaced. The deposits will be removed with an acid solution, and the disinfected circuit will then be rinsed with water. In greenhouses where the nutrient solution is recycled, the measures taken will be more important. The circuit will need to be disinfected several times to be sure to get rid of the pathogen.

The trays and boxes reused to contain the plants will be disinfected. We will be particularly vigilant about their sanitary quality. Tools used for tillage in contaminated plots should be thoroughly cleaned before use in other healthy plots. It will be the same for the wheels of the tractors. Thorough water rinsing of this material will often be sufficient to rid it of infested soil.

* 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