Apple scab control

Strategies to combat apple scab have undergone profound changes over the last 40 years in Brazil due to changes in the behavior of the fungus populations. venturia inaequalis against the application of specific action fungicides. With a serious picture of resistance problems, management measures need to be observed carefully to ensure the sustainability of these control tools.

Apple scab, caused by venturia inaequalis, is the main disease of apple trees in the producing regions of southern Brazil. It can cause losses of up to 100% in production if control measures are not adopted efficiently.

Scab control, taking into account the high susceptibility of the cultivars Gala, Fuji and their respective clones, predominant in southern Brazil, is carried out with prophylactic measures during autumn and winter to reduce inoculum, and the use of fungicides in spring it's summer.

During the more than 40 years of commercial apple cultivation in Brazil, several fungicides from different chemical groups were introduced to control scab and other diseases. Practically all of these products, with the exception of some that were withdrawn for causing harm to humans and the environment, have a protective, curative and eradicating action, and continue to be used to control scabies.

History of the use of IBEs fungicides

In the 1980s, a new phase in scabies control began with the adaptation of the Mills Table to Brazilian conditions. The introduction of Ergosterol Biosynthesis Inhibitors (IBEs) fungicides, with retroactive action on apple scab, contributed greatly to this. The first IBE fungicide marketed for scab control was triforine and then bitertanol. Soon after, the most efficient IBEs were introduced, such as fenarimol and tebuconazole and, later, myclobutanil, tetraconazole, triflumizole, cyproconazole and difenoconazole, among others.

With the introduction by the Ministry of Agriculture and Supply (Mapa) of IBEs fungicides and the implementation of Phytosanitary Warning Stations in São Joaquim and Fraiburgo (both in Santa Catarina) a new phase in scab management began. IBEs fungicides were recommended up to 96 hours after the start of PI.

However, after 10 years of use of fungicides from this group, we began to observe, especially in experimental areas, a failure to control scab. In 1992 and 1993, a survey was carried out to evaluate the frequency of isolates of V. inaequalis resistant to IBEs and also to dodine when erosion of the sensitivity of isolates of V. inaequalis to both groups of fungicides. A few cycles later, more critical results were observed, with an increase in the level of resistance and the frequency of isolates resistant to IBEs. In a more recent trial, in a greenhouse, it was shown that the efficiency of curative fungicides is below 30% when applied 96 hours after inoculation.

History of use of QoIs fungicides

From 1995 onwards, QoIs fungicides began to be tested and, in 1998, kresoxim-methylic was launched, starting the use of fungicides, also called strobilurins, in Brazil. Later, trifloxystrobin was introduced and, later, famoxadone and pyraclostrobin. Such fungicides were very important given that it was a new chemical group with high efficiency in controlling scab. In addition to the protective action and high affinity with the waxy layer of apple leaves, they acted strongly on the sporulation (eradicant action) of the fungus and also had a small curative action due to their deep action. Later, in the 2004/05 cycle, a survey was carried out across the entire producing region in southern Brazil, in which the occurrence of 4% of isolates of V. inaequalis resistant to QoIs fungicides, still providing high effectiveness of the fungicide. Over time, it began to be observed, in field trials, that the efficiency of these fungicides was no longer as good as it had been observed when these products were introduced. In the 2010/11 cycle, it was observed that kresoxim-methylic no longer controlled apple scab in field trials. A survey was then carried out in which it was found that 73,8% of the samples V. inaequalis were resistant, that is, they germinated in 20 ppm of kresoxim-methyl added to the 2% agar medium.

To confirm the results, greenhouse tests were carried out to evaluate the effect of QoIs fungicides, applied alone or in mixture with other products. According to these results, all tested QoIs failed to control scab, demonstrating cross-resistance to all fungicides in this group. Kresoxim-methyl, trifloxystrobin, picoxystrobin, pyraclostrobin and famoxadone were tested.

History of use of Dodine and Methyl Thiophanate

In Santa Catarina, dodine was intensively used in the 1970s, due to its protective action similar to other conventional fungicides, its curative effect similar to that of benzimidazoles, and its excellent eradicating action, with a suppressive effect on spore production. However, in the years 1991 to 94, when a long period of El Niño influence prevailed, several producers associated the high severity of scab with the development of resistance to dodine. Results of the survey carried out in Santa Catarina indicated that in most orchards the severity of scab was related to the management itself, however, in some orchards in the region of São Joaquim, Santa Catarina, isolates of V. inaequalis dodine tolerant in critical proportions.

A recent study, however, has shown that the fungicide dodine (45 g a.i./100 L to 54 g a.i./100 L) has good efficiency in the protective control of apple scab. It is likely that the significant decrease in its use has still maintained a sensitive population of V. inaequalis enabling some efficiency since it is quantitative resistance. In order to learn a little more about the efficiency of dodine, some field and greenhouse tests were carried out. According to these results, the resistance level appears to be stabilized and is independent of the orchard's history. In other words, dodine in a high dose showed good efficiency when tested against V. inaequalis, isolated from an orchard that is 30 years old and another that is 8 years old. It is likely that the fungus population has stabilized and become widespread throughout the producing regions.

ANP fungicides

Fungicides from the Anilinopyrimidine group have been tested since 1994 and launched together with fungicides from the QoI group. These are fungicides that have protective and curative action for up to 72 h.a.i. Despite the risk of resistance emerging, these fungicides remain very active according to tests recently carried out in the greenhouse and in the field.

Protective fungicides

Contact or protective fungicides are very important given the lack of evidence of resistance to date. However, due to the fact that they do not penetrate the plant tissues, they must be applied before rainy periods. After application, a deposit forms in the vegetative organs of the plant. The fungicide adheres to the surface to later be redistributed by rain and dew and, in this way, the product becomes inactive over time. Other factors that affect fungicides are: 1- Environment: washing by rainwater, dew, hydrolysis and photolysis; 2- Action of the microflora present on the surface of the leaves and fruits that can decompose the product; 3- Morphological characteristics of the plant that interfere with adherence and effectiveness (serosity, hairiness, etc.); 4- Fungicide formulation and 5- Phenology. The leaf emission factor is extremely important in the management of foliar diseases, given that the fungicide does not transfer from one leaf to another in sufficient quantity to control the disease.

For control to be efficient, contact fungicide replacement must be carried out at certain intervals before a new period of infection occurs and the pathogen establishes itself in the apple tree. This replacement depends on the dose, amount of rain and, mainly, the emission of new leaves. Trials carried out in São Joaquim have shown that rainfall of 30mm and 60mm does not affect the efficiency of fungicides such as Captan, Folpet and Dithianon. The protective effect of the fungicide is then more influenced by the formation of new leaves and subsequent growth of terminal branches.

In a study carried out at the São Joaquim Experimental Station, under constant temperature conditions, it was observed that it takes 6,2, 2,6 and 1,8 days to form a new leaf at temperatures of 10ºC, 15ºC and 20ºC, respectively. In other words, at an average temperature of 12,5ºC it takes 4,4 days to form a new leaf. In evaluations carried out in the field in the springs of 2008, 2009, 2010 and 2011, it was observed that during the primary scab cycle the average daily temperature was 14,2ºC and that in this condition an average of 3,8 and 4,1. 4 days to open a new leaf, in cvs. Gala and Fuji, respectively. These field results indicate that the residual interval for the action of protective fungicides, under the conditions of São Joaquim, fluctuates around XNUMX days.

Considering the occurrence of multiple resistance of V. inaequalis To IBEs, Benzimidazoles and QoIs, protective fungicides are becoming increasingly important for scab management. Therefore, it is recommended to use it in a full dose, applied as close as possible to the beginning of a rainy period.

Conclusions and recommendations

The scab control strategy has been very dynamic over the 40 years of commercial apple cultivation in Brazil due to changes in the sensitivity of apple populations. V. inaequalis against specific action fungicides. There is currently a situation of multiple resistance aggravated by the recent emergence of resistance to QoIs fungicides. Fortunately, according to preliminary studies, it has been observed that this population still maintains a relative sensitivity to the fungicide dodine. If its efficiency is proven, dodine could once again be an important tool in controlling this disease. Therefore, the use of protective fungicides applied as close as possible to a rainy period is the recommended method for controlling scab. The addition of phosphite and amino acids also helps the performance of these products. Anilinopyrimidines, with their curative action lasting up to 72 hours, can help in the most critical moments and when it is necessary to carry out curative applications. IBEs fungicides, in the absence of a better alternative, continue to be recommended as they still have action (IC of 30%) on scab and are very important for controlling powdery mildew. Finally, inoculum reduction during autumn and winter is a fundamental practice in the search for sustainability of control strategies using fungicides.

José Itamar Boneti, Yoshinori Katsurayama, Epagri/ São Joaquim

Article published in issue 79 of Cultivar Hortaliças e Frutas