How to manage postharvest rot in citrus

Fruit handling and conservation techniques have received great attention in recent years, as post-harvest damage can exceed 20%, resulting mainly from diseases. In citrus fruits collected in packing houses São Paulo, the incidence of rot exceeded 14%, after two weeks of storage at 25ºC.

Fruit discard rates are generally a reflection of the incidence of rot, since fruits are disqualified for sale due to the presence of symptoms. The probability of infection depends on the amount of inoculum present in the fruit, according to which, the amount of spores in the packing houses influenced the levels of rot. In a survey of the mycoflora in packing houses of citrus from São Paulo, a predominance of genera Cladosporium e Penicillium. However, among the important post-harvest pathogens in citrus, the genera Penicillium e Alternaria were the most frequent in these packing houses, as well as in Spanish citrus groves.

Problems arising from high levels of contamination in orchards and packing houses are increased when there are isolates resistant to fungicides. Studies have found that the phenomenon of resistance is one of the main causes of the failure of post-harvest chemical treatments. The vast majority refer to P. digitatum against benzimidazoles, sodium orthophenylphenate and imazalil. Samples of the environment packing houses São Paulo found that 39% of isolates from P. digitatum were resistant to the fungicide thiabendazole and 1% to imazalil. The high presence of P. digitatum resistant to fungicides in packing houses suggests the field as the main place for resistance to occur, probably due to the high number of fungicide applications during the production cycle.

Organic and conventional orchards

There is growing interest in agroecological farming systems, such as organic, with a large market niche to be explored. The organic citrus cultivation system has been adopted in several Brazilian states. However, the main obstacle to its production is the lack of scientific knowledge.

In organic tangerine cultivation in Spain, greater environmental mycoflora was observed than in conventional orchards, possibly related to the intensification of microbiological life in the soil and the increase in biodiversity reported in organic systems.

Recent studies in organic and conventional 'Valencia' orange orchards, located in Borborema and Itápolis, São Paulo, sought to identify and quantify post-harvest diseases, as well as characterize the environmental mycoflora and detect the presence of isolates of P. digitatum resistant to the fungicides thiabendazole and imazalil in orchards.

The total incidence of rot in 'Valencia' oranges differed significantly between the four orchards studied at 14 days of storage, with higher incidences in organic fruits compared to those from the conventional system. Higher incidences of lasiodiplodia stem rot were observed in organic fruits (15% to 20%), as well as phomopsis stem rot (Phomopsis citri) in organic fruits from the Borborema orchard (4%). In the average orchard, lasiodiplodia rot was the most common disease (10%), followed by phomopsis stem rot (2%). Green mold, anthracnose, black rot (citri alternaria), fusarium rot (Fusarium spp.), sour rot (Geotrichum citri-aurantii), blue mold (P. italicum) and aspergillus rot (Aspergillus niger) were found less frequently (

The comparatively lower incidence of rot in fruits produced in the conventional system is probably due to more intensive phytosanitary management, with the use of fungicides with mesostemic/systemic action. It is important to highlight that there are few certified input options in organic agriculture for controlling pests and diseases when compared to conventional agriculture.

The stem rot of Lasiodiplody It assumes greater importance in hot and humid regions, such as Florida and the Caribbean. The high incidence of this disease in 'Pera' oranges, in Recife, was attributed to the temperature at the fruit storage location, around 30°C, combined with the use of ethylene to degreen the fruits, which causes early abscission of the calyx. , facilitating the entry of the pathogen that is latent in this tissue.

Green mold, considered the main post-harvest disease of citrus in semi-arid regions, was not very significant in 'Valencia' oranges, as observed by other authors with fruits of the same variety destined for export, with an incidence of less than 1,0% . However, in 'Pera', 'Natal' and 'Lima' oranges, produced and sold in the state of São Paulo, green mold was the main disease, with an incidence of more than 10%, suggesting that the 'Valência' variety may be less susceptible.

The diversity of rots suggests the need for more effective control measures during the production and post-harvest phases. The use of pesticides and good agricultural practices, including adequate fertilization, cleaning pruning and removal of fallen fruit in the orchard, reduce dead plant tissue and consequently the source of pathogen inoculum. Post-harvest infections can be reduced through careful handling of fruits during harvest, transportation, processing and storage; of the use of drain for cleaning the fruits before entering the packinghouse and cleaning, with products based on chlorine and quaternary ammonia, the collection boxes and facilities of the packinghouse. Likewise, disease reduction can be achieved by using post-harvest fungicides. Refrigeration during storage and transport of fruits considerably delays the development of rot.

The environmental mycoflora did not differ between the orchards studied. Among the post-harvest pathogens with economic importance, Penicillium e Alternaria were the most frequent, similar to what was observed in Spanish orchards, packing houses paulistas and Ceagesp. No correlation was observed between the frequency of P. digitatum found in orchards in São Paulo and the incidence of mold in fruits from these orchards. Other factors such as fruit susceptibility to infection and environmental conditions before and during harvest are also important, making it impossible to establish a quantitative relationship between fungal population levels in orchards and rot damage. Environmental inoculum levels of P. digitatum em packing houses They also did not correlate with mold rates. There is no general criterion that allows us to discern the critical limits of fungal contamination above which there is a high risk of an incidence of unacceptable mold occurring. However, in packing houses, the aim is to define, using population fungal levels, dirty and clean areas and establish critical limits that allow determining the effectiveness of hygiene operations.

No isolates of P. digitatum resistant to thiabendazole+imazalyl in the environment of citrus orchards. Resistance of P. digitatum thiabendazole did not differ between cultivation systems, with an average of 47% in orchards. Resistance to imazalil was low (

Of the isolates from P. digitatum obtained from oranges sampled in the orchards, 67% from the conventional system and 61% from the organic system were resistant to thiabendazole, with no resistance to imazalil and thiabendazole+imazalil.

The low frequency of isolates resistant to imazalil is probably due to the non-use of the imidazole group in orchards, in addition to this chemical group being considered to pose a low to medium risk in the development of resistance. The higher frequency found for thiabendazole is probably due to the continuous use of benzimidazoles (methyl thiophanate and carbendazim) in orchards to control floral rot (Colletotrichum acutatum) and citrus black spot (Guignardia citricarpa), and which have a similar mode of action to thiabendazole.

The frequency of isolates from P. digitatum resistant to thiabendazole is worrying, as resistant populations could easily proliferate. The effectiveness in controlling green mold, caused by isolates resistant to thiabendazole, was lower compared to non-resistant isolates, in fruits treated with the fungicide. The combination of fungicides with different modes of action prevents or delays the development of resistant populations. However, recent work has already found a lower sensitivity of isolates of P. digitatum to the fungicides fludioxonil and pyrimethanil, even before they were used commercially.

Click here to read the article in issue 76 of Cultivar Hortaliças e Frutas.