Management measures against rust in garlic

Among the various diseases that affect garlic crops is rust, caused by Puccinia porri (Sowerby) G. Winter (syn. Puccinia there). Known since 1809, it is one of the main diseases of the crop, commonly found in all producing regions. Its greatest intensity occurs in the South and Southeast of Brazil. The disease promotes the destruction of the aerial part of the crop, which consequently leads to reduced productivity.

The fungus is biotrophic and belongs to the phylum Basidiomycota, class Puccioniomycetes and order Pucciniales. It is autoic, with a complete cycle, but rarely found in spermogonial (or pycnium) and ecium (or aetium) forms, not reported in Brazil. The urediniospores (or urediospores) (Figure 1) produced in the urediniums (or uredia; uredosorium) are yellowish in color, rounded, with an irregular surface, measuring from 23µm to 32µm x 20µm to 26µm and later the formation of dark brown teliospores measuring from 28µm to 45µm x 20µm to 26µm produced in thelia (or telia; teleutoserum) (Becker, 2004).

The disease is favored in plants stressed by lack or excess moisture in the soil. Also due to unbalanced fertilization and excess nitrogen and organic matter, or even cultivation in compacted and low soil, which favors the accumulation of water (Massola Jr, 2011).

Symptoms

 Symptoms are yellow pustules due to the production of uredospores. Under conditions favorable to the development of the disease, pustules can occupy the leaf blade, causing the leaf to dry (Figure 2). In a more advanced stage of the disease, the formation of uredospores is less and the production of teliospores occurs, which gives the pustules a dark brown or black color (Figure 3). Leaves with a high severity index can turn yellow and die (Figure 4), causing plant depletion (Figure 5), with the formation of reduced-sized bulbs (Becker, 2004; Massola Jr, 2011; Pavan et al..

Epidemiology

Temperatures between 10°C and 24°C and prolonged periods of leaf wetness favor the development of the disease, which finds ideal conditions between 16°C and 21°C and above four hours of leaf wetness. Temperatures below 10ºC and above 24ºC are unfavorable for the development of the disease. The disease is more intense when the rainfall is lower (Massola Jr, et al., 2011; Napier, 2012; Pavan et al., 2017). Wind is the main disseminator of spores, while rain contributes to the deposition of spores suspended in the air (Becker, 2004).

In an experiment conducted at the Instituto Federal Catarinense (IFC), Campus Rio do Sul, no rust was found on the onion cultivar Bola Precoce – Empasc 352 planted next to garlic with rust symptoms. The same was observed when artificial inoculation with the fungus was carried out. However, onion rust is reported in the Southeast, where the climatic conditions and/or cultivars are possibly different during the onion growing season in relation to the South of Brazil. Furthermore, there are no studies in the country on the existence of any specific physiological race between cultures.

The disease is more prevalent after the differentiation of the bulb, which leads to the assumption that the translocation of photoassimilates from the leaf to the bulb reduces the plant's resistance. Also, the decrease in leaf waxiness can favor an increase in pathogen infection.

The fungus can survive on “guaxa” plants that are normally found in crops, on bulbils and/or bulbs that were not harvested or in the surroundings of storage sheds. Green onions are a perennial host and even without showing symptoms, the fungus can maintain viability, as it is biotrophic. Work carried out at the IFC/Campus Rio do Sul found that uredospores collected from green onions caused symptoms in garlic, but not in onions. The disease is also found in leeks.

In relation to epidemiology, work was carried out by student José Carlos Kusma, from the Agronomy course at IFC/Campus Rio do Sul, under in vitro conditions to evaluate the influence of temperature and photoperiod on the germination of uredospores of P. porri. Uredospores were removed from the garlic leaves using a brush (no. 8) and washed with sterilized water. The suspension of 100µl of urediospore suspension containing a concentration of 1x105 urediospores/ml was spread with a Drigalski loop in Petri dishes containing 1% Agar-Water medium. Then, the plates were incubated in Biological Oxygen Demand (B.O.D.) germination chambers at temperatures of 3°C, 10°C, 15°C, 20°C, 25°C and 30°C (±1° C) in the dark. In a second moment, the experiment was repeated, incubating the uredospores of P. porri in D.B.O. at 16°C (ideal germination temperature obtained with the polynomial equation (Figure 6A) with photoperiods of zero, six, 12, 18 and 24 hours of light. For both experiments, the percentage of germination was evaluated after 24 hours of incubation. Germination was quantified under an optical microscope with a four-fold objective, visualizing 100 random uredospores on the plate. Those with a germ tube larger than the size of the spore were considered germinated.

Based on the results obtained, it was found that the temperature has a great influence on the germination of uredospores of P. porri. It is observed that the highest percentage of germination occurred at 15ºC with 22%, while at 10°C, 20°C and 25ºC they remained stable, without major differences (Figure 6A), varying between 9,75%, 12,5 % and 11,75%, respectively. The germination of uredospores of P. porri it is sharply reduced at extreme temperatures, where at 3ºC and 30°C the percentage of germination was only 0,5% and 0%, respectively. When the temperature went from 25°C to 30°C, there was a 96% reduction in germination. Using the equation generated by the curve (y = -0,094x² + 3,156x – 9,191; R² = 0,846) (Figure 6A), the optimal temperature for the germination of uredospores of P. porri it was 17°C.

Regarding the germination of uredospores in different photoperiods, a polynomial response of 2º was observed (Figure 6B). Using the equation y = -0,041x2 + 1,075x + 17,65 (R² = 0,169), it was found that the photoperiod most favorable to development is 13 hours of light. Therefore, it is possible that P. porri its germination is favored by longer periods of light, so spring days that increase the hours of light, such as what occurs in southern Brazil during the crop cycle, favor the germination of uredospores and the occurrence of the disease. It is concluded that the germination of uredospores was obtained at temperatures from 10°C to 25°C, with the optimum temperature being 17°C and 13 hours of light.

Disease management

Management practices are essential to reduce the damage caused by the disease. The pH must be recommended for the crop, as it promotes better plant development. Annual applications of lime raise the pH, which can benefit the imbalance of micronutrients, favoring infection.

It is recommended to avoid excess nitrogen fertilization and follow what is indicated by the soil analysis. Excess nitrogen makes tissues more succulent, favoring penetration and multiplication.

It is beneficial to control soil moisture by draining the area and/or raising the beds, in order to avoid humidity and increase the period of leaf wetness for the plant.

Irrigation must be carried out according to the crop's needs, avoiding the accumulation of water in the crop. Prevent the crowding of plants, as this facilitates dissemination and also increases humidity inside the plants.

Avoid the use of agricultural implements, as well as the transit of agricultural machinery and people who have passed through contaminated areas, in order to reduce the spread of uredospores. Eliminate any “guaxo” garlic plant that remains vegetating in the field and around storage sheds.

Make use of cultivars such as Caiano Roxo, Gigante de Lavínia and Centenário, as they are more resistant to the disease (Massola Jr, et al., 2011). Azevedo (1997) found that the Contestado and Quitéria cultivars had a lower number of pustules per leaf, as well as a lower number of uredospores produced, while Caçapava, Caçador and Cará recorded higher ones. The Dourados and Peruano cultivars were intermediate.

Regarding alternative control, Becker & Marcuzzo (2007) found a 20% reduction in the severity of the disease through the weekly application of 10% propolis solution. However, when adhesive spreader was added, it caused phytotoxicity in the crop.

The use of resistance inducers was evaluated by Lunardelli (2003), who found that acibenzolar-S-methyl had a similar result to the conventional use of Mancozeb, 250g/100L. However, the dose of 6g.pc/100L caused phytotoxicity to the plants. The same author found that the use of bioflavonoid + ascorbic acid in 300ml p.c/100L provided a reduction of 30% to 68% in the severity of rust. The use of calcium + ascorbic acid and potassium 20 (N3%+K2O). The use of potassium 20 provided a 20% increase in the aerial part. Ecolife and potassium 20 provided, respectively, 11% and 10,5% increase in bulb weight.

On the Agrofit page of the Ministry of Agriculture, Livestock and Supply (Mapa) there are 43 fungicides registered to control the disease, 17 of which are based on mancozeb and another 13 based on tebuconazole. Becker (2004) found almost 90% control in relation to the control with the use of mancozeb and that its efficiency was also confirmed by Garcia et al. (1994), but they found that the pesticide did not differ from propiconazole and triadimenol. However, copper oxychloride, in this work, showed low efficiency. In Ethiopia, propiconazole showed greater control effectiveness compared to mancozeb (Mengesha et al., 2016).

Dalla Pria et al. (2008) found that the mixture of trifloxystrobin and tebuconazole at 75g/ha and 150g/ha in the SC and WG formulations was adequate to control the disease due to its similarity with tebuconazole and because it did not cause visible damage to the crop. The use of tebuconazole and azoxystrobin provided good protection against rust when sprayed at ten-day intervals in California (Koike et al..

The aforementioned measures aim to promote disease management and reduce damage caused to the crop. 

Leandro Luiz Marcuzzo, Instituto Federal Catarinense, Rio do Sul Campus

Growing Vegetables and Fruits December/January 2021

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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