Rhizoctonia solani: Protective treatment for cotton

Seedling damping off, caused by Rhizoctonia solani, is widespread in Brazil, mainly in the cerrados of Mato Grosso, Mato Grosso do Sul, Minas Gerais, Goiás and Bahia

29.07.2016 | 20:59 (UTC -3)

The cotton crop is attacked by a large number of fungal diseases. With the increase in cotton planting area in Brazil, a significant increase in phytosanitary problems has been observed, mainly those related to the occurrence of diseases in the initial phase of crop development.

Losses in yield caused by the incidence of diseases in plants vary, among other factors, depending on the crop, the type of pathogen, the location, environmental conditions, the susceptibility of the cultivar and the control measures used. Considering these aspects, it is difficult to say to what extent a plant could tolerate a disease without suffering significant losses in productivity. The intensity of the loss in production is generally determined by the time in which the infection occurs and the affected organ in the plant.

If, on the one hand, cotton farming has been guaranteeing profits for its producers, on the other, many still need to weigh the risks of this activity before venturing into planting. The incidence of diseases is among the factors that cause the most damage to the most unsuspecting producers and, if not efficiently controlled, can result in incalculable crop losses.

Initial cotton diseases, mainly those caused by Rhizoctonia solani, such as seedling tipping and mela, are widely disseminated in Brazil, mainly in the cerrado regions of Mato Grosso, Mato Grosso do Sul, Minas Gerais, Goiás and Bahia, causing significant damage in the initial phase of crop establishment, due to the reduction of the plant population and, sometimes, due to the need for reseeding – which is always very expensive, representing 6,44% of the production cost. The levels of damage caused by these diseases are dependent on several factors; however, under Brazilian conditions, what has most favored its occurrence is the cotton monoculture associated with intensive soil preparation (which facilitates the dispersion of the fungus), which often favors flooding and waterlogging situations, contributing to an increase in pathogen inoculum potential in the area. The use of seeds with low vigor, associated with planting at times favorable to the occurrence of this disease, is also a factor that predisposes to the attack of R.solani that must be considered.

Of all the diseases that attack the cotton plant, “falling over” is considered one of the main ones, which can cause great losses, mainly related to failures in the stand, which can lead to the need for reseeding.

This disease is caused by a complex of soil and seed fungi, whose species can vary greatly from place to place. However, the main causal agent of seedling collapse in Brazil is Rhizoctonia solani Kuhn, anastomosis group (AG)-4 (teleomorph: Thanatephorus cucumeris (A.B. Frank) Donk), due to the frequency it occurs (more than 95% of cases) and the damage it causes in the initial phase of crop establishment.

This disease is widely spread in Brazil, mainly in the cerrado regions (where 85% of the cotton grown in Brazil is found) in the states of Mato Grosso, Mato Grosso do Sul, Minas Gerais, Goiás and Bahia. It occurs at the seedling stage (post-emergence damping-off) and attacks the seeds during germination (pre-emergence damping-off).

To get an idea of ​​the importance that this disease assumes in this context, some data collected about losses due to its occurrence are surprising. In California, according to official data, around 1991 tons of cotton were lost annually, from 1993 to 12.733, due to tipping. In the USA, in 1995, a reduction in cotton productivity due to initial diseases was estimated at around 180 thousand tons. In California, this same year, these losses were estimated at 17.850 tons, greater than those recorded in previous years. In the last ten years, in the USA, loss estimates have revealed average values ​​of 2,8% per year. To date, no data of this nature has been collected in Brazil.

the fungus Rhizoctonia solani

R. solani It is a necrotrophic parasite, a natural inhabitant of the soil. It is a polyphagous fungus, as it attacks several plant species. R.solani It can be transmitted by seeds, however, this rarely occurs, which is why the seed is not considered the main source of inoculum for this pathogen. This fungus, being present in the soil and/or seeds, in addition to causing significant losses in the seedling phase (stand failure), can also serve as an inoculum for subsequent crops.

Symptoms are initially characterized by the wilting of the leaves and subsequent tipping of the seedlings. This fungus causes depressed, reddish-brown lesions on the neck and roots of cotton seedlings.

Of all the recommended practices for controlling initial cotton diseases, seed treatment with efficient fungicides plays a primary role, being considered, to date, the main measure to be adopted and the safest and most economical option (represents only 0,17. XNUMX% of the total production cost) to minimize the negative effects of these diseases.

This is an essential practice when reducing the quantity of seeds during sowing, with a view to eliminating the thinning operation, being recognized throughout the world as one of the most effective and convenient measures, becoming increasingly widespread and adopted in integrated control schemes for cotton diseases.

Fungicides and seed treatment

Each year, several fungicides are tested with the aim of verifying their efficiency in controlling these diseases. The performance of these products depends on the population of these fungi in the soil, that is, it is influenced by the pathogen inoculum pressure in the soil and also by interactions with other fungi, which may demonstrate biological control. Likewise, the susceptibility of cultivars may also influence the benefits of seed treatment with fungicides.

On the other hand, the benefits of treating cotton seeds with fungicides are less evident in areas where the pathogen inoculum density is relatively low or when soil moisture and temperature conditions are ideal for rapid germination and emergence. However, it must be considered that, to date, there is no evidence that the use of fungicides in seed treatments with specific action against R.solani can be dispensed in areas with a history of occurrence of this pathogen.

It should be noted that the main effect of seed treatment with fungicides is observed in the initial phase of crop development, that is, at most between ten and 12 days after emergence. During this period, there is efficient protection of the cotton plant, obtaining adequate populations of plants due to uniformity in germination and emergence. However, it is also necessary to highlight that, if the climatic conditions are favorable after this period of protection, some fungi may settle on the cotton seedlings – which is normal – as a result of the loss of residual power of the fungicides, not meaning that the treatment it was inefficient.

The main objectives of treating seeds with fungicides are to eradicate or reduce, to the lowest possible levels, the fungi present in the seeds; provide protection for seeds and seedlings against soil fungi; prevent the development of epidemics in the field; promote uniformity in germination and emergence; provide greater sustainability to the crop by reducing risks in the crop implementation phase and promoting the initial establishment of the crop with an ideal population of plants.

The combined action of systemic fungicides with protectants has been one of the most effective strategies for controlling seed and soil pathogens, since the spectrum of action of the mixture is expanded by the action of two or more products. In this way, there are better seedling emergence in the field and better damping-off control rates with the use of available mixtures, compared to the isolated use of a certain fungicide. It should also be noted that, with the use of mixtures, the emergence of resistant populations among pathogens is largely avoided.

Until recently, the use of fungicides was exclusively aimed at controlling phytopathogens. With the launch of strobilurins and the evolution of this group of chemical products, the concept of control gained new perspectives, due to the proof of direct influences arising from the use of these products on physiological processes in uninfected plants. This activity was called “physiological effect”, known in the vegetative and reproductive phases, moving from the pure and simple concept of seed treatment to a concept of plant robustness.

Chemical treatment of seeds with fungicides, from the point of view of integrated disease management, is one of the simplest, low-cost methods and results in highly positive impacts on increasing crop productivity.

When the environmental issue is analyzed, there is also the advantage of not altering the biology of the soil, as the amount per hectare is minimal, being quickly diluted and degraded in the soil. Furthermore, among other pesticides, fungicides are those that have the least negative impact on the environment. When compared with other control practices (foliar spraying = product distribution over 10.000m2/ha and granules in the planting furrow = application in 500m2/ha), seed treatment with fungicides has the advantage of the amount of product used corresponding to the application in just 55m2/ha (which means a localized application of low doses/ha).

Table 1 shows the efficiency of different fungicides in controlling seedling damping caused by R.solani. The test was conducted in the greenhouse at Embrapa Agropecuária Oeste, in Dourados, Mato Grosso do Sul. Seeds not treated and treated with fungicides were sown in sand contained in plastic trays, arranged in individual holes, equidistant and 3cm deep. Inoculation with R.solani AG‑4 was made using 5g of the fungus inoculum/tray, distributed homogeneously on the surface of the substrate. The fungus was cultivated for 35 days in autoclaved black oat seeds crushed in a mill (1mm). A significant effect of the fungicide treatment was observed on the initial and final emergence of seedlings, as well as on the control of pre- and post-emergence damping-off of the cotton plant, with the best results being obtained by the treatments tolylfluanid + pencycuron + triadimenol and azoxystrobin + fludioxonil + mefenoxan, followed by carboxin + thiram, PCNB and pencycuron. The least efficient fungicide was carbendazim + thiram. The fungus R.solani it can cause pre- and post-emergence tipping, which was observed under the conditions of the present test. The evaluation of the percentage of final seedling emergence (26 DAS) reflects the efficiency of fungicides in protecting against the attack of R. solani, as well as the maintenance capacity of the stand, in order to avoid post-emergence tipping caused by this pathogen. The drastic effect of the pathogen can be observed when comparing the results obtained in controls with and without inoculation. The best results in relation to the studied variables were obtained with the use of fungicide mixtures (with the exception of the carbendazim + thiram treatment) compared to the isolated use of a certain product, which can also be seen in Table 1. The results observed demonstrated the phytocompatibility with cotton of all the fungicides used, with no symptoms being observed in the seedlings that would reveal the presence of phytotoxic effects resulting from the use of these products, such as the appearance of seedlings with deformed and twisted leaves or even delayed emergence. Related to this last aspect, it must be considered that sowing was carried out under ideal conditions of soil temperature and humidity and also at an adequate depth (3cm to 5cm).

Table 1 - Initial and final emergency and pre- and post-emergency tipping caused by Rhizoctonia solani depending on the different fungicide treatments applied to cotton seeds. Embrapa Agropecuária Oeste. Dourados, MS

Treatments

Dose a.i./100kg

Seedling emergence (%)

Seedling damping (%)

de

cements

Home

Final

Pre-emergence

Post-emergency

Tolylfluanid+pencycuron+triadimenol

100 + 75 + 40

90,0 to

88,5 b

2,2 and

1,7 d

Azoxystrobin+fludioxonil+mefenoxan

30 + 5 + 15

90,0 to

88,0 b

2,2 and

2,2 d

Carboxin+thiram

140 + 140

89,0 to

83,0 c

3,4 d

6,7 c

PCNB

250

89,0 to

82,5 c

3,4 d

7,3 c

Pencycuron

75

88,0 to

82,0 c

4,3 c

6,8 c

Carbendazim+thiram

90 + 210

75,0 b

66,5 d

18,9 b

11,3 b

Uninoculated witness

-

92,0 to

92,0 to

0,0 f

0,0 and

Inoculated witness

-

47,0 c

29,5 and

49,2 to

37,2 to

Media

-

82,50

76,50

10,42

9,15

CV (%)

-

10,31

9,08

15,82

16,40

Treatments

Dose a.i./100kg

Seedling emergence (%)

Seedling damping (%)

de

cements

Home

Final

Pre-emergence

Post-emergency

Tolylfluanid+pencycuron+triadimenol

100 + 75 + 40

90,0 to

88,5 b

2,2 and

1,7 d

Azoxystrobin+fludioxonil+mefenoxan

30 + 5 + 15

90,0 to

88,0 b

2,2 and

2,2 d

Carboxin+thiram

140 + 140

89,0 to

83,0 c

3,4 d

6,7 c

PCNB

250

89,0 to

82,5 c

3,4 d

7,3 c

Pencycuron

75

88,0 to

82,0 c

4,3 c

6,8 c

Carbendazim+thiram

90 + 210

75,0 b

66,5 d

18,9 b

11,3 b

Uninoculated witness

-

92,0 to

92,0 to

0,0 f

0,0 and

Inoculated witness

-

47,0 c

29,5 and

49,2 to

37,2 to

Media

-

82,50

76,50

10,42

9,15

CV (%)

-

10,31

9,08

15,82

16,40

Averages followed by the same letter in the column do not differ significantly from each other. (Duncan, 5%).

Click here to read the article in issue 179 of Cultivar Grandes Culturas.

Mosaic Biosciences March 2024