Management of purple spot in organic garlic cultivation

The production of organic garlic demands knowledge and technologies that guarantee competitiveness and coexistence with diseases such as purple spot, caused by the fungus Alternaria porri

15.05.2018 | 20:59 (UTC -3)

In general, purple spot on garlic, caused by the fungus Alternaria porry (Ellis) is one of the diseases that most compromise crop yield in Brazil. Considered to be widespread in all garlic producing regions, it causes production losses that can reach 50% to 60% (Zambolim et al.

In the organic system of the Incaper Agroecology Reference Unit, garlic is one of the crops that participate in the management system adopted since 1991 and, despite this system providing a biological balance in the soil and increasing plant resistance to pathogens, they are still observed high levels of intensity of occurrence of a.porri in production fields (Souza et al, 2011). For this reason, protective products such as Bordeaux mixture and lime sulfur have been used throughout the crop cycle.

The initial symptoms of the disease appear in the form of small white spots that quickly develop a clear center. As they increase in size, the spots become zoned and typically purple in color, surrounded by a chlorotic halo that extends up and down the leaves. Under favorable conditions, the lesions become covered with the dark fruiting structures of the pathogen. The leaves turn yellow and dry from the apex, reducing their photosynthetic area, and new leaves are emitted at the expense of the bulb's reserves, resulting in the production of small bulbs (Nunes & Kimati, 1997; Jaccoud Filho et al, 1985). The progress of the infectious process and the increase in susceptibility are associated with the increase in the age of the plants and the beginning of the fruiting period. During this phase, there is a greater demand for sugars and nutrients for the formation of bulbs, to the detriment of foliage, which favors the infectious process in exporting organs (Rotem, 1994).

High humidity is the most important environmental factor for the development of the disease, as the fungus is dependent on water for spore germination and sporulation on the plant surface. The fungus can grow at temperatures ranging from 6°C to 34°C, but the optimal range of development is between 21°C and 30°C (Nunes & Kimati, 1997; Zambolim & Jaccoud Filho, 2000).

The experiment

Knowing the right time for the incidence of a disease and employing effective control at the right time can promote an increase in crop productivity (Pinto & Maffia, 1995). In Figure 1 it is possible to observe the level of damage that alternaria can cause to organic garlic crops in a few days, comparing the healthy field at 100 days (on the left) and the fully infected field at 140 days (on the right). Thus, the objective of this work was to evaluate the influence of the time at which the incidence of Alternaria porry on the vegetative and productive performance of organic garlic, to support the definition of the appropriate control moment.

The test was carried out at the Incaper Agroecology Reference Unit, in Domingos Martins, state of Espírito Santo, at 950m altitude. Garlic was planted on 25/4/2011 and harvested on 13/9/2011, totaling a cycle of 141 days. The Gigante Curitibanos cultivar was used, multiplied in the organic system itself for 21 years, planting the bulbils in beds 0,20m high, 1,20m wide, with a spacing of 30cm between rows and 15cm between plants, representing an area useful capacity of 7.000m2 per hectare.

Broadcast planting fertilization was carried out, at a dose of 15t/ha of organic compost (dry weight), incorporated into the soil before planting and, subsequently, top dressing was carried out with 400ml/m-2 of liquid biofertilizer applied via soil, fortnightly, from 60 days until the beginning of bulbification. In addition, weeding was carried out manually whenever necessary.

Experimental evaluations began on 13/8/2011 (110 days), marking randomized blocks, with five replications, containing experimental plots formed by 20 useful plants. The treatments consisted of four phenological patterns (Figure 2), according to the beginning of the disease incidence in the field, as follows:

1 - Start of incidence 125 days after planting (Level 1 - low);

2 - Start of incidence 120 days after planting (Level 2 - medium/low);

3 - Start of incidence 115 days after planting (Level 3 - medium/high);

 4 - Start of incidence 110 days after planting (Level 4 - high).

The evaluations were carried out at harvest, adopting the commercial classification of bulbs with diameters equal to or greater than 30mm.

Analyzing the production of green biomass, commercial productivity, average weight and average diameter of bulbs, it was found that there were significant effects for all incidence periods (Table 1). On the other hand, the incidences of alternaria at 125, 120 and 115 days did not cause different effects on the percentage of commercial bulbs, pseudostem diameter and bulbar ratio. Only the 110-day season differed from the others, reducing the number of commercial bulbs by 30%, the diameter by 51% and the bulbar ratio by 35%, respectively, in relation to the 125-day season.

Table 1 - Garlic productive characteristics related to the start days of incidence of Alternaria Porri. Incaper, Domingos Martins, 2011

Onset of infection (days)

Green Biomass

(g)

Commercial bulbs

(%)

Productivity

 

(kg ha-1)

Average Weight (g)

Diameter (cm)

Bulbar ratio

Bulb

Pseudo-

stalk

125

2.073 to

100 to

8.657 to

55,65 to

5,46 to

1,09 to

0,20 to

120

1.684 b

100 to

7.335 b

47,15 b

5,17 ab

1,00 to

0,19 to

115

1.439 b

100 to

6.082 c

39,10 c

4,86b

0,94 to

0,19 to

110

673 c

70 b

2.837 d

26,03 d

4,03c

0,53 b

0,13 b

Onset of infection (days)

Green Biomass

(g)

Commercial bulbs

(%)

Productivity

 

(kg ha-1)

Average Weight (g)

Diameter (cm)

Bulbar ratio

Bulb

Pseudo-

stalk

125

2.073 to

100 to

8.657 to

55,65 to

5,46 to

1,09 to

0,20 to

120

1.684 b

100 to

7.335 b

47,15 b

5,17 ab

1,00 to

0,19 to

115

1.439 b

100 to

6.082 c

39,10 c

4,86b

0,94 to

0,19 to

110

673 c

70 b

2.837 d

26,03 d

4,03c

0,53 b

0,13 b

1 Means followed by the same letter in the column do not differ from each other, using the Tukey test at 5% probability. 

The effects of incidence seasons on green biomass and, consequently, on active leaf area, caused significant reductions in productivity and average bulb weight, for all disease incidence seasons. In Figure 3, it is observed that the reductions increase with the earlier incidence of the disease in the field.

Organic garlic – Box

The growth of the agroecology and organic agriculture sector has demanded knowledge and technologies that enable greater competitiveness of organic products in the market. In this context, the phytosanitary management of diseases, through correct diagnosis and preventive actions, is the safest and most rational path to follow for sustainable agriculture, where we seek to obtain adequate productivity, with less environmental impact, especially in the cultivation of garlic in a system organic, where this issue stands out as one of the most relevant factors.

 Click here to read the full article in issue 77 of Cultivar Hortaliças e Frutas. 

 


 

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