Milder temperatures lead to a greater occurrence of macadamia diseases

Scientists from Embrapa and Universidade Estadual Paulista (Unesp) revealed the main microorganisms that cause diseases in macadamia crops. Field and laboratory tests recorded three main illnesses, all caused by fungi: raceme drought, caused by Cladosporium that affects inflorescences; canker/gummosis caused by Lasiodiplodia on stems and branches and anthracnose, caused by Colletotrichum, which promotes deterioration in fruits. Of these, only Lasiodiplodia benefited in warmer environments, the other two pathogenic microorganisms developed better in mild temperatures.

Although these fungi already occur in Brazil in other cultures, this is the first time that these species have been associated with macadamia nuts. Raceme drought and canker/gummosis were considered the most potentially important, as they prevailed and could cause, respectively, a reduction in the number of fruits or death of young plants. In racemes, it was found that flowers close to opening or already open are more likely to be infected.

The results obtained under laboratory conditions indicate that there are active ingredients capable of inhibiting the development of each of the culture's pathogens, and there are even other molecules that could also be used. However, the efficiency of the active ingredients is specific to each of them.

According to Rosicléia da Silva, in her master's thesis at Unesp, it was proposed to report the incidence of fungal pathogens associated with samples of symptomatic macadamia organs, which resulted in a collection of phytopathogens associated with the plant organs of the nut and the relationship between the agent causal and the seasonal period in which they occurred.

Furthermore, the sensitivity of the agents to fungicides and the effect of different temperatures on the development of phytopathogens were also evaluated, which is important information to define which diseases have the greatest potential to cause an impact, and the times and conditions conducive to their occurrence. This work generates subsidies for practices to be used in the integrated management of each one. For this, leaves, sections of branches and trunk and inflorescences with possible symptoms of disease or signs of fungal colonization were collected periodically, from 2019 to 2021, in Dois Córregos (SP).

main diseases

Macadamia cultivation has been expanding and, therefore, information of this nature can mitigate damage caused by diseases and their possible economic impacts, as the available literature describes diseases that occur in other countries. With support for correct diagnosis, producers and professionals in the area will be able to avoid possible mistakes in management and the consequent inappropriate recommendation of control measures. Another aspect to consider is that knowledge of which pathogens occur in the country is also important to prevent the introduction of others that are still absent or in specific planting areas.

Although there are reports of microorganisms associated with macadamia nuts in Brazil, there is scientific proof of their occurrence so far for only two disease-causing agents - the fungi Lasiodiplodia theobromae, which causes cracks in the stem with occasional gum exudation, which can lead to death. mainly young plants and Neopestalotiopsis clavispora, an occurrence observed in Vitória da Conquista, Bahia, causing irregular light brown leaf spots on the leaves and disease severity estimated at around 25%.

“Although in the literature there are reports of Cladosporium and Lasiodiplodia as causal agents of diseases, the results indicate that the species we found are different from those already described. In Australia, the country where it originates, the most common causes of disease in flowers are different species of Botrytis, Neopestalotiopsis and Cladosporium. However, in the work we carried out, only one species of Cladosporium was able to reproduce symptoms of raceme drought and no incidence of Botrytis was observed in any of the samples examined. Furthermore, another pathogen considered important in plantations abroad, Phytophthora, was not found associated with any sample”, highlights Embrapa Meio Ambiente researcher Bernardo Halfeld Vieira.

The development of a disease is the result of the interaction between a susceptible host, a pathogenic agent and favorable environmental conditions. Of these three components, the environment is the one that most frequently presents changes and influences its progress throughout the cycle of a culture. In general, the critical climatic factors that affect the development of diseases in plants are temperature, relative humidity and rainfall. In the region where the study was carried out, the average monthly temperature ranged from 19°C to 28°C and the average monthly relative humidity ranged from 42% to 85%, however, during the period, there was a low frequency and volume of monthly precipitation.

The results and information suggest that the causal agents of raceme drought and anthracnose in fruits are favored by milder temperatures. In laboratory tests, the temperature that led to the fastest growth of these pathogens was 23°C, a condition close to those that occurred when Cladosporium prevailed in the area.

On the other hand, for Lasiodiplodia, higher temperatures around 30°C favored its development.

Considering resources involving chemical control, according to Agrofit (2023), there are some registered active ingredients: acibenzolar-S-methyl, chlorothalonil, fluxapyroxad + pyraclostrobin, thiabendazole, copper hydroxide and azoxystrobin + difenoconazole – all indicated for the control of anthracnose.

“The entire set of information generated, from the identification of each pathogen, the conditions conducive to the development of these fungi, the times in which they prevailed and the sensitivity of each one to different active ingredients can support the appropriate management of each disease”, explains Kátia Nechet, researcher at Embrapa Meio Ambiente.

A large part of the state's production is sold to the QueenNut Macadamia processor, in Dois Córregos, SP. According to Leonardo Moriya, Queennut technician, knowledge about the diseases that occur in the crop and the choice of cultivars is of fundamental importance for the formation of a productive orchard. It must be done according to the climate, soil, density, topography and management method. The most used varieties in Brazil are those from the Agronomic Institute (IAC) and the Hawaii Agricultural Experiment Station (HAES). Orchards in Brazil are considered young, the first commercial plantations appeared in Limeira, SP between 1976 and 1980, so information on the occurrence of diseases in the crop is still scarce.

Another important measure for marketing is the so-called recovery rate, which represents the percentage of almonds obtained after processing, an indicator of the effectiveness of pest and disease controls and harvest and post-harvest procedures. World macadamia consumption is growing at a rate of more than 8% per year, making it the nut with the greatest expansion in consumption. Following the trend of the previous ten years, its global production continued to increase, with the 2020 harvest reaching more than 62 thousand tons of almonds, an increase of 4% compared to the previous season and 45% higher than the average in the 10 previous years.