Challenges in managing brown rot in peach trees

One of the biggest concerns in global fruit farming is post-harvest diseases, especially those acquired by quiescent infections, which cause major losses in the commercialization of fruits.

Although the peach tree is susceptible to several diseases, brown rot is the main one. Responsible for quantitative and qualitative losses of peaches, in all producing regions of Brazil and the world.

The disease is caused by the fungus Monilinia fructicola (Wint) Honey [asexual phase Monilia fructicola (Wint.) Honey], the main disease of stone fruit plants, occurs in practically all orchards, causing severe losses, especially in years with high rainfall. It also infects plum, cherry, apricot and almond trees, from the family Rosaceae and the genre Prunus L.

The losses caused to production will depend on the degree of susceptibility of the cultivar, the aggressiveness of the pathogen, climatic conditions and management measures employed by producers.

Climate conditions can have indirect effects on the progress of this disease, by directly interfering with the host's development. The host may be weakened by unfavorable environmental conditions, thus becoming more susceptible to the pathogen.

Cycle of host-pathogen relationships

Brown rot is a polycyclic disease, its spread occurs through the dispersion of conidia through the air and increases continuously from the appearance of the first infected fruit until its harvest.

This fungus belongs to the class Discomycetes, which are ascomycetes with apothecium, whose characteristic is the formation of sexual ascospores, inside a structure called ascus, and asexual conidia with blastic conidia, formed in a chain and with an average size of 8-28 × 5- 19μm (most 12-16 × 8-11μm) and hyaline.

The fungus also produces well-developed sclerotia that determine its survival in winter and, when they germinate, they form apothecia where asci are produced. From mummified fruits partially buried in the ground, which emerge during flowering time. In mummified plants and fruits they can also survive in the form of dormant mycelium.

The asci measure 102-215x3-13μm and the ascospores measure 6-15x4-8μm, which are formed in apothecia, typical of the order Hetotiales. Through this structure, the ascospores are projected and disseminated by the wind, constituting the primary inoculum of the disease.

In Brazil, the natural occurrence of the sexual phase of the pathogen is rare, and can be found more frequently in the southern region of Rio Grande do Sul (Figure 1). This can be explained by the requirement of particular climatic conditions. But the most common form found in the field in all producing regions is the asexual form.

In the climatic conditions of Pelotas, it is possible to visualize the occurrence of the perfect phase in a peach orchard, after a sequence of cold and rainy days.

Figure 1 - Fruits with brown rot caused by the fungus Monilinia fructicula in a peach orchard, in the climatic conditions of Pelotas.

The survival of this pathogen from one harvest to another occurs in mummified fruits, peduncles, withered flowers and branches with cankers. In regions where sexual reproduction structures (apothecia) are observed, they also help with survival by germinating from mummies that have fallen and been slightly buried in the ground.

Conidia are spread by wind, water and insects, reaching susceptible parts of the plant, mainly flowers and fruits, and in the case of fruits, conidia can penetrate through the cuticle or wounds, colonizing them quickly the closer they are. is at the maturity stage. Climatic conditions are also important for the development of the imperfect phase.

disease symptoms

The pathogen infects flowers, pre- and post-harvest fruits and branches, and can destroy fruit production in a few days, if environmental conditions are favorable and control measures are not adopted.

During flowering, there is burning of petals and canker on the branches (Figure 3), and during fruiting, fruit rot occurs pre- and post-harvest, being more severe in fruits close to maturity.

Conidia and ascospores are capable of infecting flowers and immature fruits during early spring. In flower infection, there may be necrosis of the anthers, ovary and peduncle, which can kill them. The flowers turn brown, wither, and may exhibit sporulation of the fungus and remain attached to the branch by an exuded gum. This infection can cause significant losses in production due to a reduction in the number of flowers, impairing effective fruiting, in addition to serving as a source of inoculum during the fruiting period. Furthermore, under unfavorable climatic conditions, flowers that survive infection tend to produce fruits with quiescent infection, causing damage during the harvest and post-harvest periods.

Quiescent infection increases linearly with increasing relative humidity during the flowering period and increases exponentially with fruiting and in the late stages of fruit development.

The detection of quiescent infection makes it possible to estimate in advance the incidence of the disease in the period prior to harvest, helping to establish control strategies and adopt appropriate methods of storing and marketing the fruits.

Fruits can be infected with Monilinia spp. at any stage of its development, but the disease only becomes serious when the fruit begins to ripen. At the beginning of maturation, when the mechanical resistance of the epidermis decreases, the resistance of green fruits can be broken by mechanical or physiological damage, making them susceptible to the pathogen. During pre-harvest, initially small brown lesions are observed on the fruits, with a soggy appearance, which evolve into extensive brown spots covered by the sporulation of the fungus. Then, the fruits begin to dehydrate, becoming mummified, remaining on the plant or in the soil.

The occurrence of environmental conditions favorable to the disease during the fruiting period favors fruit rot while still on the plant, causing mummified fruits that can serve as a source of inoculum.

The pathogen's conidia are produced in periods of high relative humidity (above 80%) and temperatures of 20ºC - 25ºC. It can germinate at temperatures above 10ºC and below 32ºC (optimal temperature 25ºC). Apothecia are formed at a temperature of 15ºC. When environmental conditions are favorable, the pathogen can complete its cycle in a few days. Temperatures above 33ºC are unfavorable for peach brown rot. Therefore, in regions with a temperate climate, the amount of inoculum is much greater, which is why disease epidemics are much greater.

disease control

The fundamental cultural control practices are: the elimination of inoculum sources represented by diseased branches and mummified fruits. Recommendations for control are based on preventive treatments through the use of fungicides. This chemical control must be carried out by spraying the plant in the critical phases of the crop (flowering and from three weeks before fruit ripening). Preferably opt for systemic fungicides, rotating different active ingredients, to avoid the emergence of populations of fungi resistant to fungicides.

To reduce the disease epidemic, it is essential not only to remove the sources of primary inoculum, but also to reduce the number of conidia on the surface of the fruits.

It is extremely important to monitor the types of damage occurring pre-harvest and at the time of harvest, to predict the potential for fruit loss due to brown rot, additional care during harvest, such as cleaning containers with sodium hypochlorite at 0,5 .XNUMX% or other type of sanitizer. Cold storage after harvest also helps to reduce losses during marketing or industrial processing.

The use of implements in the lines reduces the production of apothecia. This orchard management can influence the reduction in the production of the perfect phase of the pathogen.

When using chemical control, necessary care must be taken, such as the use of Personal Protective Equipment (PPE) and compliance with the withdrawal period for each fungicide. It is also important that the sprayers are well adjusted and calibrated so that the application results in good coverage of the fungicides on the plant, especially on flowers and fruits.

Click here to read the article in Cultivar Vegetables and Fruits 82.