Wicked rot

In onion cultivation, several diseases attack the root system and the aerial part, causing losses throughout the crop cycle. Among these diseases, one of the most significant, both in the field and in storage, is crown and bulb rot caused by the nematode Ditylenchus dipsaci.

The nematode D. dipsaci (Kühn) Filipjev belongs to the class Secernentea, order Tylenchida, suborder Tylenchina, superfamily Tylenchoidea, subfamily Anguininae, family Anguinidae. This phytonematode was detected in the country in 1979, initially in garlic crops. Due to the expansion of new cultivation areas and the production of supervised bulbils not being sufficient to meet this demand, mainly in Santa Catarina, several producers acquired bulbils in commerce that would be used for consumption and that would be contaminated with the nematode, resulting in its wide dissemination (Tihohod, 1993).

The life cycle of D. dipsaci varies from 19 to 23 days, at 15ºC, until reaching the adult stage. It leaves the egg two days after oviposition and during this cycle it goes through four ecdyses, one of which occurs while still in the egg (Figure 1A, B). Mating is necessary for reproduction and the female begins oviposition four days after becoming an adult. Each female lays around eight to ten eggs per day for 25 to 50 days, reaching a total of 200 eggs to 500 eggs during the 45 to 73 days that they can live (Tihohod, 1993).

The survival of phytonematodes varies depending on the species, however, Ditylenchus dipsaci It is one of the few that survive through anhydrobiosis, when it reduces its metabolic activity and can remain dormant for up to 23 years (Wordell Filho, 2006).

Weed species such as Gnaphalium spicatum (white macela), oxalis corniculata (Clover), Amaranthus deflexus (caruru) are considered hosts while bidens pilosa (black picão) e Galinsoga ciliata (white picão) are of little importance in the survival of the nematode (Fonseca et al.

However, in Alto Vale do Itajaí, due to the availability of area for onion cultivation, the crop is normally sown without rotation in the same area, which makes it possible for the nematode to survive in the soil, on host weed plants and on crop plants. onion “guachas” that remain in the field (Figure 1H).

Contamination of onion plants occurs through the penetration of the nematode, inhibiting seed germination (Figure 1I, J, K) or into the crown and bulb tissue below the soil surface (Figure 1B). The nematode colonizes the internal tissues of the root, bulb, pseudostem, and can reach the leaves (Figure 1C, D, E). Pectolytic enzymes are essential for the dissolution of the middle lamella and for the establishment of parasitism within the scales (Figure 1F), which ends up darkening the tissues due to the action of phenols (Wordell Filho, 2006). In farming, the nematode ends up being spread through irrigation and rain water and soil movement due to the traffic of machines, equipment and people.

The Alto Vale do Itajaí presents some outbreaks of the disease and it is assumed that it may have been introduced in two ways: through producers in the region, who also grow onions in the mid-west of Santa Catarina and who, by using areas previously cultivated with garlic, may have contaminated machines and implements used in soil preparation, introducing the nematode into the region; by contaminated onions brought from these regions by producers or traders to be stored and/or processed and the disposal being deposited in cultivation areas in the Alto Vale do Itajaí.

This information describes the classic symptoms caused by D. dipsaci, in addition to disease management measures.

Symptoms

Symptoms during germination end up delaying seedling development. At this stage, the nematode is attracted to the cotyledon, which, after invading, thickens the tissue and topples the seedling.

The attacked plants have thickened, spongy pseudostems and chlorotic, twisted leaves. The release of new leaves occurs at the same point, presenting a duster or brush appearance. These leaves become flaccid, causing the entire aerial part to fall over within two to three weeks and forming lumps. in crops.

In the bulb, the external scales rupture, causing rot to occur due to the penetration of bacteria and, as a result, releasing a strong odor. The bulb internally appears spongy with a mealy appearance, in which the internal scales are loose (Figure 1G), facilitating invasion by bacteria that end up decomposing the bulb in the field or in storage.

Nematode management D. Dipsaci

D. dipsaci It has the potential to cause 100% damage to the crop, which can occur in the field or in storage. Therefore, preventive measures are extremely important (Table 1)

Table 1 - Preventive measures against D. dipsaci

Although some producers use bio-based products, there is no concrete information regarding their agronomic effectiveness confirmed by research. In the literature, to date, there is no information on the control of this nematode through biofumigation and soil solarization.

For this disease in onion crops, there is no registered agrochemical, but with the concern generated by the damage, producers end up using products indiscriminately, especially after the disease is detected. Furthermore, with the use of these agrochemicals, other problems may occur, such as phytotoxicity to the crop, nematode resistance to the active ingredient and contamination of the bulbs.

FIGURE 1 – Life cycle of Ditylenchus dipsaci in onion. (Adapted according to Agrios, 1988 and Tihohod, 1993)

FIGURE 2 – Symptom of twisted leaves with a duster-like appearance due to the presence of D. dipsaci

FIGURE 3 – Symptom of peeling and cracks in the bulbs resulting from D. dipsaci

FIGURE 4 – Symptom of spongy and mealy scales infected by D. dipsaci

Box – Onion culture

The onion (Allium cepa L.) is one of the most widespread cultivated plants in the world, considered the second vegetable in global importance, surpassed only by the tomato (Song et al, 2007). Originally from Asia, it was introduced to Brazil by the Portuguese and is cultivated from the South to the Northeast, ranking third among the most economically important vegetables in Brazil (Boiteux & Melo, 2004).