Deadly stain

Leandria leaf spot, also known as Cucurbit Zoned Spot, is a disease caused by the fungus

, identified and characterized in Brazil by Eugenio Rangel in 1915, in his article “Some fungi from Brazil, new or poorly known” in partnership with André Maublanc. The name Leandria is a tribute to the Brazilian botanist Leandro do Sacramento.

Among the biotrophic diseases, the disease can be considered the most severe and destructive in cucumber crops, assuming great importance in conditions of high temperatures and high humidity. It is widely distributed in producing regions, occurring throughout practically all of Brazil, as long as the conditions for its development are met. The aggressiveness and virulence of the pathogen is high, destroying the crop within one to two weeks. The disease can easily be confused with bacterial angular spot, anthracnose, alternaria spot or scab, especially in its early stages.

Leaf Symptoms

The pathogen produces white necrotic lesions with defined faint brown margins, visible on both sides of the leaf, from 5 days after infection. The main affected crop is cucumber, although it causes damage to chayote, melon, watermelon, pumpkins and nigauri (São Caetano melon) and can survive in other cultivated or wild species of cucurbits. In the case of cucumber, the initial lesions appear angular and can be confused with those of bacterial angular spot, caused by Pseudomonas syringae pv. lachrymans. Initially, older leaves are affected, progressing to younger leaves. The lesions coalesce later, affecting the entire leaf area. Unlike bacterial angular spot, Leandria spot lesions do not present a characteristic anasarca, although initial symptoms appear in the form of small waterlogged areas. Furthermore, the veins inside the lesion are dark in color, giving the spots a slightly reticulated appearance.

There are no visible symptoms on the stem of affected plants or on the petiole of leaves.

In the melon tree, the symptoms manifest themselves as small chlorotic spots with a reddish-brown necrotic center. Subsequently, these spots coalesce, giving the lesions a blackish appearance.

In watermelon, symptoms appear as dark spots distributed along the leaf blade. These spots have a white dot in the center and irregular edges.

Description of the causal agent

The conidia are spherical, pigmented, multicellular, made up of 7 to 18 globose, agglomerated cells, which give them a typical globular and muriform shape, with a brown color. They are produced on the lower surface of the leaves, accompanied by a hyaline fungal mycelium and conidiophores with 3 to 6 septa. Rangel described the conidium of the fungus as a “strange, almost black fruiting”, multilobed, with beautiful club-shaped hyaline conidiophores, with lobed contours. In more practical terms, as a complementary diagnosis, a brown cranberry-like cluster is found under the common microscope, which characterizes the conidium.

Particular characterisitics

The conditions for increasing the rate of disease progress are high humidity and high temperatures. Under these conditions, each cell germinates individually, as reported by KUROZAWA and PAVAN. The fungus spreads naturally through wind and water. Little is known about its ability to survive in crop remains. Still in the case of cucumber, the name Net Spot, in English, for the zoned spot, is due to the formation of a slight lacework in the holonecrotic tissue, conferred by the ribs inside the lesion. This is possibly the characteristic that differentiates it from other foliar diseases.

Control strategies

Control strategies should focus on reducing the initial inoculum and interfering with the rate of disease progression. Therefore, the following measures are suggested, in order of importance:

1. Choice of resistant cultivars. No cases of immunity to the disease have been reported. In the case of cucumber, some materials in the Aodai line incorporate a certain resistance. The Kinsei-T hybrid, of the Japanese type, has vertical resistance, providing excellent escape resistance to the plant;

2. Crop rotation. Normally, rotation with plants from other families tends to reduce the pathogen inoculum potential;

3. Elimination of cultural remains and alternative hosts.

chemical control

There are no specific recommendations for chemical control of the disease. KUROZAWA and PAVAN propose the use of benzimidazoles and triazoles to control the disease. Cañizares points to the use of methyl thiophanate, alone or accompanied by chlorothalonil, as a control measure. Some formulations of chlorothalonil, methyl thiophanate and tebuconazole are registered for controlling Leandria leaf spot in cucurbits.

Jorge Massaki Hasegawa

Sakama seeds

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