Septoria cucurbitacearum Sacc., (1876)

Septoria leaf spot

- classification : Fungi, Ascomycota, Dothideomycetes, Dothideomycetidae, Capnodiales, Mycosphaerellaceae
- téléomorphe : Mycosphaerella
- English names: Septoria leaf spot

Septoria cucurbitacearum is responsible for septoria, a disease that is rather widespread in the world. It is widely present in several states of the United States (Delaware, Massachusetts, New Hampshire, New York, Pennsylvania, Wisconsin, Illinois), in Europe (Bulgaria, Denmark, France, Germany, Hungary, Italy, Portugal, Romania), in USSR and Lithuania, Africa (Ethiopia, Kenya, Zambia), Australia

Although reported in France , this disease does not seem to be rampant in our territory at the present time.

Septoria affects the leaves and fruits of a more or less extensive range of Cucurbitaceae: melon , cucumber, bitter melon, calabash, pumpkin, summer, winter and pepper squash, watermelon, Bryonia dioica (= B. cretica ) ...

Its damage is sometimes significant, in particular on species of the genus Cucurbita .


Main symptoms

Septoria causes roughly comparable symptoms on the organs of all Cucurbits.

Small moist spots, dark brown and 1-2 mm in diameter, appear on the leaves . Subsequently, they spread rather slowly and take a more or less circular shape and a beige to almost white tint in dry conditions. These spots are surrounded by a narrow brown border, and the central tissues sometimes crack. Some authors report the presence of a yellow halo surrounding the spots.

Note that a few tiny brown to black globular structures are visible within the altered tissues, especially on the leaves; it is pycnidia of S. cucurbitacearum which characterize its parasitism. On fruits, these fruiting bodies do not seem to form easily. In their place, secondary fungi are observed which can lead to a misdiagnosis. So beware.

Note that S. cucurbitacearum is responsible for small whitish canker lesions (1-2 mm in diameter) on fruits of Cucurbita moschata, C. pepo and pumpkin. They are raised and have a reddish to white tint depending on their stage of development and the state of maturity of the fruits. Let us add that this fungus has also been associated with rots on Cucurbita maxima and C. moschata , in association with other microorganisms. Its implication in this type of rot does not seem obvious.


Biology, epidemiology

• Conservation, sources d'inoculum

S. cucurbitacearum perdure plus d'une année sur les débris de cultures grâce à son mycélium dormant et/ou ses chlamydospores. Il doit aussi pouvoir se maintenir sur le matériel utilisé pour la culture.
Il se transmet par les graines, ces dernières une fois contaminées assurent sa conservation et sa dissémination.

• Pénétration, invasion

Des conidies issues de pycnides se déposent à la surface du limbe ou des fruits. Elles germent et le mycélium pénètre par les stomates, puis envahit les tissus. Des symptômes sont déjà visibles au bout de quelques jours.
 

• Sporulation et dissémination

Subsequently, S. cucurbitacearum forms pycnidia in the tissues from which fine and long septate conidia are expelled as mucilaginous cirrhia. These spores are dispersed by splashing from rains or sprinkler irrigation. Workers and equipment moving through wet vegetation also spread spores from plant to plant. Contaminated seeds could contribute to the conservation and spread of the disease over long distances.

• Conditions favorable to its development

Climatic periods characterized by precipitation, high humidity, and temperatures between 16 and 19 ° C are optimal for infections and the development of septoria.


Protection methods 

• During cultivation

As soon as the very first symptoms appear, and if the plot is located in a production area where septoria is recurrently serious, fungicide treatments should be carried out fairly quickly. Several active ingredients provide effective protection, such as copper oxychloride, chlorothalonil, maneb and mancozeb ...

Note that this disease does not seem to occur in France and that no active ingredient is currently authorized for this use.

In addition, the presence of free water on the plants and high humidity should be avoided as much as possible. For that, it is necessary to ventilate the shelters as much as possible, and to ban watering and irrigation by sprinkling.
In the open field, sprinkler irrigations will not be implemented if they are not essential. Otherwise, they will be carried out in the morning or in the morning so that the foliage wears off quickly, in no case in the evening.

Severely affected leaves, fruits and plants will be removed from crops and destroyed. At the end of cultivation, the plant debris will either be eliminated or buried deeply.

• Next crop

Healthy seeds should be used. If in doubt, their disinfection or coating with one or more fungicides should control S. cucurbitacearum .

Crop rotations of at least 2 years will be considered. In addition, avoid setting up a new crop near plots of Cucurbitaceae already affected.

It may also be recommended to plant with densities which subsequently allow good aeration of the vegetation, good wiping after rains or sprinkler irrigation, and to produce rather vigorous plants. Avoid planting in hydromorphic soils. It will be better to irrigate the plants by drip rather than by sprinkling. The equipment used should be disinfected between each culture.

During wet and cold periods, careful monitoring of the crops will detect the first symptoms and trigger chemical protection if necessary. The fungicides previously reported will be used.