Disease spreader

There are many plant diseases caused by organisms from the oomycete group (oomycetes). Oomycetes, in the past, were classified as fungi. The diseases caused are given the common names of downy mildew, white rust, seed rot and root rot, causing damage to various crops.

This group of microorganisms belonged to the fungi kingdom, however, more recently it was classified in the Stramenopila kingdom. The ancient classes of fungi Hyphochytridiomycetes, Labyrinthulomycetes and Oomycetes are part of this new kingdom.

The main plant species attacked by oomycetes, the common names of the diseases and the scientific name of the causal agents involved are presented in Table 1

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In general, foliar diseases caused by oomycetes have the common name of mildew. This is because the signs of the disease develop mainly on the underside of the leaves made up of the set of conidiophores (sporangiophores) that emerge through the stomata. Due to the presence of this cottony mass, they receive the common name of mildew.

On the other hand, the name white rust is due to the fact that the signs are similar to those of “rust”, but white in color.

Although fungicide is conceptualized as a chemical substance that kills fungi, today it must be taken into account that as oomycetes are no longer classified as fungi, there is still no specific designation for the chemical substances used to control them. Logically, they should be popularly called “oomycetics” (which kills oomycetes). Although there is still no clear definition regarding the use of this specific terminology, in this text the word fungicide will also be used, as that substance that is toxic, including to oomycetes.

In some crops, the greatest damage caused by oomycetes is related to the reduction of plant population, seed rot, seedling death and root rot. In these cases, one of the recommended control measures is the treatment of seeds with efficient fungicides (oomyceticides) to control Pythium spp. how they capture it and metalaxyl. These compounds have been used to treat corn and soybean seeds to protect them from attack by Pythium present in the soil.

Although all diseases caused by oomycetes are important, mildews deserve attention due to the damage and loss they cause. Control of these diseases is efficiently achieved through the use of fungicides.

Among them, the most used are fungicides (“mildiocides”) from the cupric group [based on copper oxychloride, copper hydroxide, curprose oxide and basic copper sulfate (Bordeaux and Viçosa mixtures)].

They can also be controlled by the fungicides chlorothalonil and mancozeb. Of these, those based on chlorothalonil are more efficient.

However, a specific group of fungicides, highly specific to downy mildews, has been used as the most potent weapon for its control. It should be noted that this group of fungicides does not control diseases caused by true fungi. In this group, the fungicides cymoxanil, benalaxyl, dimethomorph, fenamidone, metalaxyl, propamocarb and zoxamide deserve to be highlighted. Due to the specificity of this group, it is generally used in pre-made mixtures or tank mixes to avoid the development of resistant breeds and even to broaden its spectrum of action. The most frequently mixed protective fungicides are chlorothalonil, maneb, and mancozeb.

In all these cases, fungicides are applied to aerial organs to prevent infection or kill the newly established parasite.

In the case of root rot of citrus plants, caused by Phytophthora spp. control has been achieved by foliar spraying of the fungicide fosetyl aluminum. The product is absorbed by the leaves and translocated via the phloem to the plant's root system, controlling the fungus infecting the root system. In this case, this is a unique example in phytopathology of the translocation of a fungicide via the phloem, that is, applied to the leaves controlling a parasite in the root system. It is argued that the product does not have fungicidal action “in vitro” but rather “in vivo” and that this fact may be related to its involvement in the activation of the plant's defense mechanisms.

Criteria indicating when to apply fungicides to control mildew and white rust:

When fungicide is applied to a crop, the objective is to control a disease caused by a fungus and, in this case, by oomycetes. It is accepted that the disease causes damage and loss and that the application of the fungicide will guarantee an economic return on investment.

What should you know about when to apply fungicide to control oomycetes in aerial organs?

The criteria available for technical decision-making are discussed here considering: the price of the product (plant organ harvested and sold), the cost of control, the economic return and, finally, the economic and ecological sustainability of the agricultural activity.

By concept, preventive or protective control is one in which the fungicide is applied before the deposition of the inoculum at the infection sites. Infection has not yet occurred and therefore the amount of disease is zero in leaflets, leaves, plants and in the crop. Preventive or protective control occurs when the fungicide is applied before infection. Infection comprises the subphases of spore deposition, germination, penetration and establishment of parasitism. It is likely that in some situations the application will have to be made in the vegetative phenological stages of the crop and therefore require several applications.

Without systematic monitoring, it is not known how much disease there is in the crop at the time of application, with the risk that its intensity may have exceeded the action threshold (AL).

It consists of applying the fungicide when the infection has already occurred without the presence of symptoms. Once applied, the fungicides kill the oomycete inside the tissues, and it stops developing. There was infection, but the symptoms are not visible.

Without systematic monitoring, the amount of disease at the time of application is not known, and there is a risk that the LA has already been exceeded.

This is when the fungicide is applied and detectable lesions already occur. Some fungicides kill the fungus even at this stage, when it stops sporulating.

When applying a fungicide, one should know how much disease is occurring at that time in the crop, as there may be a risk of having exceeded the LA.

Another possibility consists of applying fungicides to control the target disease at a phenological stage. By concept, the amount of disease is not taken into account and therefore it can be preventive (protective), curative or eradicant. Preventative is zero disease, curative is post-infection before symptoms and eradicant is post-symptom, by concept.

In this case, the presence or absence of the disease, its intensity, nor the cost of control and economic return are not considered.

In any situation, when applying a fungicide to a crop, one should know how much disease occurs at the time of control. Therefore, without systematic monitoring, there is no idea of ​​the intensity of the rust.

Disease onset is an indefinite or subjective term. Any intensity can be the start. The beginning of the quantity or intensity of the disease must have a value, a number, and this is possible through the scientific method.

Phytopathometry is the process of quantifying diseases and quantity of disease is synonymous with intensity of disease. Intensity can be quantified or expressed in numbers, using the criteria of incidence, severity and number of lesions per leaflet or area unit. The most sensitive method of phytopathometry is the incidence on individuals, then on leaves and finally on leaflets.

Intensity can be expressed as severity, which is the proportion of the leaf area covered with lesions, or as the number of lesions per leaflet or per cm2.

For example, the action threshold (AL) is the onset of the disease, but it is expressed as a numerical value.

Another aspect that can be considered is the amount of rainfall and the removal of residue from protective or residual fungicides. For example, a rainfall of more than 13 mm removes the deposit of protective or residual fungicides from the surface of the tomato plant. Therefore, after this event occurs at this intensity, treatments must be restarted.

Disease warning systems can also be used as criteria to indicate the time for the start and interval of crop inspections. These systems have practical application mainly in vegetable and fruit growing.

Disease prediction systems are based on the presence, time and space of three disease-determining factors: host, pathogen and favorable environment. Few prediction models are based on monitoring the inoculum and relating it to the beginning of disease development because they are very laborious. Most available systems are based on the climatic requirements for inoculum multiplication and infection.

The climate prediction model is based on the effect of the environment on the infectious process. Once the spores are deposited at infection sites, they respond to stimuli that are signals from the environment. The main stimuli are the presence of liquid water and temperature. The response of spores to wetting is mandatory and irreversible, reacting to the stimulus through germination. The climate model predicts infection (germination, germ tube growth, host penetration and establishment of parasitism). An infection is successful when wetness lasts long enough, at an average temperature, to initiate colonization of the host. This interaction is called critical period (PC). Therefore, after a PC occurs, the leaf surface can dry and the pathogen will continue its life cycle.

Most climate model-based forecasting systems use PC to predict infection. As in a population, fungal spores do not respond synchronously in germination to the duration of wetness and temperature, there are different proportions of infection and consequently the intensity of the disease, depending on their response to those environmental factors. Each individual has a requirement for duration of wetness and temperature to successfully complete the infection.

On the other hand, different intensities of disease can be obtained using different concentrations of inoculum, or different temperatures and/or durations of leaf wetness. The interaction of the duration of leaf wetness with the average temperature during this period is the cornerstone of disease prediction systems based on the effect of climate on the infectious process called fundamental or inductive climate model, which predicts infection.

The effects of interactions between temperature and duration of wetness on infection, obtained under controlled conditions, may present distortions in the field. In this case, the factors that compromise the efficiency of infection in a way that does not exactly reflect what occurred under controlled conditions are: (a) variations in temperature fluctuation in the canopy, (b) small interruptions in the duration of wetness, (c) different inoculum availability (density), (d) spore viability (% germination), (e) host predisposition (leaf age), (e) presence of nutrients and pesticides in the phylloplane, and (f) antagonistic biological activity of the organisms residing in the phylloplane. Therefore, the absolute frequency of infection cannot be predicted.

Future severity is a function of the infection efficiency of the spores, which in turn is governed by the PCs.

The PCs are different for each pathosystem, so the PC tables are also different for each species of fungus in a given host.

Prediction systems are available for the control of mildews on potato, tomato, onion and vine (www.quanta-agro.com).

UPF

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