Corn leafhopper and the transmission of diseases that affect productivity

The Corn Leafhopper Dalbulus maidis It has been considered one of the main pests of corn crops. It is no longer an exclusive pest of seed-producing regions and is now present in practically all areas where the cereal is grown, both in the 1st and 2nd Corn Harvests in the country. Losses caused by stunting and viruses transmitted by leafhoppers can reach more than 90%, especially when using a hybrid sensitive to the stunting complex.

Corn leafhopper

 The Leafhopper, in addition to causing injuries as a sucking insect, is responsible for indirect damage that generates more significant losses in the crop, through the transmission of phytopathogens such as mollicutes, Phytoplasma (Maize bushy stunt phytoplasma) and Spiroplasm (Spiroplasma kunkelii), these being the causal agents of corn stunting, and Fine Riad (Maize rayado fine-MRFV) (OLIVEIRA, 2008).

 In the adult stage, the leafhopper measures 3,7 to 4,3 mm in length and is straw yellow in color (Figure 1). It has two black circular spots on the front and is often found on the corn stalk. The yellowish egg has an embryonic period of around nine days, being placed within the plant tissues, preferably in the central vein of the leaf.

The nymph, which lives inside the corn cartridge (Figure 2), goes through five instars, a period that lasts around 17 days (MARTINS et al., 2008). Its biology is significantly affected by temperature, and at temperatures below 20°C the nymphs do not hatch (WAQUIL, 2004).

Insect life cycle

According to Oliveira (2000), one of the characteristics that make infestations more frequent by D. maidis in corn crops, it is the fact that the species has a high biotic potential and the ability to migrate over long distances. The insect can colonize from newly germinated fields to flowering, depending on the progression of insect generations, and the entry of other adult leafhoppers, especially when there are adult plants nearby.

Corn stunts

In corn cultivation, there are two types of stunting, which are caused by pathogens from the mollicute class. Pale stunting is caused by the pathogen Spiroplasma kunkelii, and red stunting is caused by Maize bushy stunt phytoplasma (OLIVEIRA et al., 2002).

The distinction between the two types of stunting is made based on the plant's symptoms: pale stunting, the symptoms initially are broad discolored, yellow or lime-green stripes at the base of the infected leaves, and later, the new leaves show the same symptom, in addition, the infected plant may present shortening of the internodes (Fig.3), malformed ears (Fig.5), deformed or absent and deformations in the tassel, however in some cases, symptoms may be mild or even absent, as the red stunt can cause symptoms such as: reddening of older leaves (Figs 6 and 8); shortening of internodes, abnormal tillering and development of several spikelets (Fig. 7) (WAQUIL, 2004).

Mollicutes invade systemically and multiply in the vascular tissues of plants, more specifically in the phloem (Fig. 4), and are transmitted to healthy plants through the vector D. maidis, which, when feeding on diseased plants, acquire the pathogen, in a latent period that varies from 3 to 4 weeks (WAQUIL, 2004). However, this period may vary depending on environmental conditions such as temperature, with the optimum temperature for the acquisition and transmission of mollicutes being between 18 and 30 °C, and temperatures below 16 °C, which may affect transmission efficiency and increase this period; and the availability of host plants (OLIVEIRA et al., 2007).

                

Fine Riad

This virus can cause production reductions of around 30%. As this virus is transmitted by the same insect vector that transmits the causative agents of red stunt and pale stunt, it generally occurs simultaneously with these diseases. However, its incidence varies in different areas and in different years, generally without reaching the same incidence levels as stunting.

The first symptoms appear as small chlorotic spots at the base and along the veins of young leaves (Figs 9 and 10). They become evident with a large number of chlorotic spots, which merge, taking on the appearance of short stripes. In general, the first symptoms of this virus appear in young plants in the field, around 30 days after sowing, and remain visible even in plants in the production phase. Stripe symptoms can best be discerned by holding the infected leaves up to the light. Infected plants may have ears and kernels smaller than normal size.

These 3 pathogens may be present in corn separately or even occur simultaneously, making diagnosis based on symptoms difficult, requiring molecular analysis to be carried out using techniques such as PCR, where the DNA sequence of each phytopathogen is compared. .

Other Damages

In addition to the damage previously reported, such as: reduction in plant size, reduction in leaf area, phloem obstruction, poor formation of ears and grains, the Stunting Complex causes ear rot, directly affecting the weight and quality of the grains, increasing the rate of stem breakage, making the harvesting operation difficult and, as a result, there is a significant loss of productivity.

Management and Control

Classification of KWS hybrids regarding tolerance to stunting complex

WORK CONDUCTED IN CANDIDO MOTA – SP. SAFRINHA 2019 WITH HIGH CIGARETTE PRESSURE