Management of fungi in solanaceous and cucurbitaceae

Difficult to control, the fungus Phytophthora capsici requires a combination of management measures to reduce disease levels

26.06.2020 | 20:59 (UTC -3)
Cultivar Hortaliças e Frutas

The Solanaceae botanical family comprises approximately 85 genera distributed throughout the world, being particularly abundant in the Americas. Many species of this family are economically important as vegetables. Frequent examples reside in the various species of Capsicum e solanum, of which it is possible to highlight peppers and peppers (C. annuum L.), chili and Tabasco peppers (C. frutescens L.), chili peppers and habanero (C. chinense Jacq.), the finger pepper (C. baccatum L.), eggplant (Solanum melogena L.), the jiló (Solanum aethiopicum var. gilo) and tomato (S. lycopersicum L. = Lycopersicum esculentum Mill.). Among these vegetables, tomatoes and peppers stand out in terms of economic importance.

Another botanical family with several important representatives as vegetables is the Cucurbitaceae family. Among the vegetables belonging to this family, melon (Cucumis melo L.), watermelon [Citrillus lanatus (Thunb.) Matsun et Nakai], the pumpkins [Cucurbita maxima Duchesne, C. moschata (Duchesne) Duchesne ex Poiret, c pepo L. and several interspecific hybrids], cucumber (Cucumis sativus L.) and the gherkin (cucumis anguria L.).

Due to the succulent constitution of vegetables, which facilitates the development of fungi and bacteria, diseases are constant challenges for horticulturists. They are normally caused by bacteria, fungi, nematodes and viruses. They can also be caused by abiotic factors, such as deficiency or excess of nutrients, phytotoxicity due to agrochemicals and inadequate light. In this case, they are also known as physiological disorders. However, fungi and pseudofungi are by far the most numerous group among vegetable pathogens and, probably, the most important.

Among vegetable diseases, those caused by soil organisms stand out, such as oomycetes of the genus Phytophthora. They cause diseases that lead to great losses because they are difficult to control. There are three species of Phytophthorthat cause diseases of economic importance in vegetables, P. infestans (tomato and potato late blight), P. capsici P. nicotianae (root and fruit rot of various vegetables).

the species P. capsici causes wilt or late blight of peppers, which can infect part or all of the plants in a crop. This pathogen also causes wilting and fruit rot in other solanaceous vegetables and cucurbits. It is a polyphagous pathogen, widely distributed in cultivated soils in Brazil and many other countries. It attacks the plant from the infested soil and is difficult to control.

HOSTS

Phytophthora capsici It has several hosts, most in the Solanaceae and Cucurbitaceae families. Among the main ones are peppers, peppers of the genus Capsicum, tomato and eggplant. Among cucurbits, pumpkins, cucumbers and watermelons stand out. Many invaders are also attacked by this pathogen and this is of epidemiological importance, as they maintain and even multiply their inoculum in the soil. In addition to these, some perennial plants are also considered hosts of P. capsici. Table 1 lists some of the main species of host plants for P. capsici, reported in Brazil and other countries.

SYMPTOMS

Symptoms of diseases caused by P. capsici in their hosts, they depend on the plant's growth stage and environmental conditions, especially temperature and the occurrence of free water in the plant via rainfall or irrigation. In peppers and peppers, the disease is sometimes called wilt or blackleg, especially when symptoms are characterized by root and neck rot (blackleg) and plant wilt. In the field, symptoms of plant wilt are generally observed in small patches (spots of withered or dead plants). However, in greenhouses you can see large patches of diseased plants or even all withered or dead plants. These symptoms occur in conditions of little free water supply, such as in dry growing regions or seasons. Under conditions of high relative humidity, and especially heavy and frequent rains, fruit rot and leaf burn, sometimes called late blight, may also occur. Attacked pepper fruits and peppers, under conditions of high humidity, show a whitish growth over the lesions. This whitish growth consists of fungal structures such as mycelium, sporangiophores and sporangia. When P. capsici attacks plants in the first stages of growth (seedlings), it can also cause tipping over.

In tomato, P. capsici It can cause problems at all stages of plant development, such as tipping over, root and root rot, wilting and fruit rot, especially in creeping tomato fruits, where it causes the deer's eye symptom. In adult tomato plants, however, the symptom most often is underdevelopment and yellowing of the plant, as tomato plants are relatively more resistant than peppers.

In addition to tomatoes, peppers and hot peppers, this oomycete causes root and fruit rot in other nightshades such as eggplant and jiló. In several regions of Brazil, severe infection of the fruits of these solanaceae is common during the rainy season, even affecting post-harvest fruits.

In cucurbits, P. capsici It causes seedlings to fall, root and stem rot, plant wilting and fruit rot. These fruit rots can occur in the field or post-harvest, causing great damage to the entire production chain of these vegetables. Typically, pumpkin and zucchini plants are more susceptible than cucumber and watermelon plants. However, the fruits of all these species appear to be equally susceptible. The disease is also more severe in rainy and hot seasons.

Wilt and blackleg symptoms on pepper plants, caused by Phytophthora capsici.
Wilt and blackleg symptoms on pepper plants, caused by Phytophthora capsici.

THE PATHOGEN AND FAVORABLE CONDITIONS

Phytophthora capsici It survives in the soil mainly in the form of oospores, since in the form of sporangia or zoospores this oomycete has a very short lifespan in this environment. However, a residual level of inoculum can survive in crop residues colonized between two harvests, leading to severe epidemics in the subsequent year if conditions are favorable. Both groups A1 and A2 have already been found in nature in Brazil in solanaceae and cucurbits, but both groups have rarely been found in the same crop. This oomycete can also survive on volunteer plants or on some invasive species.

Spread in the field occurs via irrigation or rainwater and through agricultural implements. Within a crop, the inoculum can also be spread by wind, from sporulating lesions on fruits, branches and leaves. Water splashes (rain or irrigation) can carry contaminated soil particles to the lower fruits of staked tomato, eggplant or jiló plants, causing infection. At long distances, the spread can be through infected seedlings, and there are no proven cases of the pathogen spreading via seeds. Prolonged periods of rain, temperatures of 22ºC to 20ºC and poorly drained soils are favorable conditions for the disease. The fungus attacks plants at any stage of development and penetrates through natural openings or wounds. About five days to eight days later, symptoms appear. The disease is polycyclic, that is, more than one cycle occurs in the same growing season, these being shorter and more frequent the more favorable the environmental conditions, especially temperature and humidity.

Pepper screen with 100% wilted plants, due to attack by Phytophthora capsici.
Pepper screen with 100% wilted plants, due to attack by Phytophthora capsici.

DISEASE MANAGEMENT

As there are still few commercial materials resistant to the disease for sale in Brazil, control of pepper late blight must be carried out through a combination of measures that together have an additive effect in reducing the final levels of the disease. Furthermore, even in resistant genotypes of Capsicum, resistance generally only manifests itself in adult plants, compromising the planting of peppers and peppers in areas infested by P. capsici. Finally, even with the use of partially resistant commercial materials, high levels of attack by P. capsici may occur if environmental conditions are favorable.

In cucurbits such as pumpkins, pumpkin, zucchini, cucumber, melon and watermelon there are some sources of resistance with good levels of horizontal resistance to root rot and plant tipping caused by P. capsici. However, as it is a resistance governed by a few or many genes, the transfer of this characteristic to commercial cultivars is difficult for plant breeders to carry out. Therefore, seed companies do not have much interest in this type of resistance and there are no commercial cultivars of resistant cucurbits. For fruit rot in cucurbits, there are no or very limited good sources of resistance.

Tomato seedling damping off, caused by P. capsici.
Tomato seedling damping off, caused by P. capsici.
Rot in pumpkin fruit, caused by P. capsici.
Rot in pumpkin fruit, caused by P. capsici.

To control diseases caused by Phytophthora spp. In vegetables, some farmers have used fungicides, the most common of which is mefenoxam. However, the use of this fungicide should be alternated with other products that also have an effect on oomycetes. In other countries, after a few years of intensive use of mefenoxam, resistant strains were selected, making it ineffective. When this occurs, control efficiency is compromised. However, in Brazil, the frequency of isolates of P. capsici resistance to mefenoxam is still very low, perhaps because it is still little used to control this pathogen in solanaceae and cucurbits, because it is expensive or because the commercial product is offered in a mixture of multiple active ingredients. There are no chemical products registered with the Ministry of Agriculture, Livestock and Supply (Mapa) for control of P. capsici in cucurbits. In the case of peppers, there are few registered fungicides, the majority being from the cupric group plus chlorothalonil and mancozeb. Some products based on dimethomorph, propamocarb and cymoxanil are also registered, which are specific and highly efficient against oomycetes, such as P. capsici. However, these fungicides will only have high control efficiency in the aerial phase of the disease (late blight and fruit rot) in peppers.

Another measure to manage diseases caused by P. capsici in vegetables consists of avoiding planting in soils infested by the pathogen or subject to waterlogging, especially clayey and compacted soils. It is also recommended to avoid planting during hot and rainy times of the year and, when doing so, the beds should be higher, to reduce soil humidity close to the plant's neck. The time between irrigation events (per furrow) must be increased; use drip irrigation, with the water emitter away from the plant’s neck; employ healthy seedlings and use straw as organic soil cover; avoid dense plantings and excess nitrogen fertilization. Rotate crops, preferably with grasses; Avoid planting solanaceae and cucurbits in succession in one area. It is also important to control boring insects, which make holes in the fruits and are a gateway for the pathogen to penetrate and cause rot.

Ailton Reis, Embrapa Hortaliças

Growing Vegetables and Fruits January 2020 

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. In the April/May edition you can also see: 

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