Transmission of maize streak virus from grasses to maize

MOSES RAMUSI AND BRADLEY FLETT, ARCGRAIN CROPS INSTITUTE, POTCHEFSTROOM

Maize streak virus is economically the most important foliar disease of maize in Africa and belongs to the genus Mastrevirus, which is in the family Geminiviridae.

Maize streak virus is widely distributed throughout sub-Saharan Africa. In South Africa, maize streak virus is prevalent in parts of Limpopo, KwaZulu-Natal, the Northern Cape and Mpumalanga, particularly in areas experiencing high rainfall and temperatures.

Reports by the International Service for the Acquisition of Agri-biotech Application (ISAAA) indicated maize streak virus to be an increasing problem to Africa’s subsistence farmers as many plant susceptible, traditionally open-pollinated maize varieties.

Maize streak virus is thought to have evolved from grasses and transmitted to maize plants by virus-infected leafhoppers.

Symptoms

Plants infected with maize streak virus develop symptoms 10 to 14 days after infection. Symptoms appear mostly on young growing tender leaves, which are preferred by leafhoppers for feeding (Photo 1).

However, this does not mean that the virus cannot infect older leaves. Symptoms vary from severe narrow, broad to mild chlorotic streaks centred on secondary and tertiary leaf veins and are uniformly distributed on the leaf surface (Photo 2).

Parallel yellow streaks also appear and the plants remain stunted, producing small ears. Infection increases where susceptible crops or alternate hosts are continuously available under favourable environmental conditions such as high rainfall and temperatures.

Transmission of maize streak virus

Like many other viruses, maize streak virus depends on insect vectors for transmission between host plants. Maize streak virus cannot be transmitted through seeds or any other method. Cicadulina species are the only insects known to transmit maize streak virus from one maize plant to another.

Overwintering of the virus and vectors occur primarily in grasses and areas with hydromorphic soils or tropical environments with irrigation where maize can be grown during the dry season.

Leafhoppers feed on many species of grass and also use them for oviposition. Some species of grasses, such as Setaria barbata (Lam.) Kunth and Brachiaria lata (Schumach) Hubbard, harbour both the vector and the virus.

Female leafhoppers lay eggs onto the living tissue of the host, which might be dormant or develop and hatch within a few weeks. After hatching, the nymph feeds on vascular tissues. Infected leafhoppers transmit the virus from symptomless grasses to maize host plants.

In areas where maize is grown throughout the year, both maize streak virus and the vector could survive on maize and/or grasses. Once infection has occurred, increased spread of the virus will occur over time. Maize streak virus causes severe losses if maize is infected at an early growth stage, approximately three weeks after emergence. Severe infection can result in 100% yield losses, depending on time of infection and plant growth stage when initially infected.

Cicadulina mbila and C. chinai leafhopper species are able to accumulate and transmit the virus. Difference in virus accumulation is that the vector (C. mbila) accumulates the virus over a period of time whereas C. chinai quickly attains saturation level.

Maize streak virus is found in the head, thorax and abdomen of both active and inactive transmitters after feeding. Leafhoppers have sucking mouthparts, which enable them to penetrate the plant cell wall either by mechanical force or by using salivary and gut enzymes.

Vector insects are capable of obtaining the required amount of virus, which even after bypassing through the gut to the haemolymph, transmission could still occur. However, in some vectors the gut serves as the first physical barrier for movement of viruses towards the haemolymph, with the salivary glands being the second transmission barrier.

The gut may be considered as the major storage compartment for viral particles, with the haemolymph being only a transit compartment. Regular increase in the amount of viruses during sustained feeding of C. mbila on diseased plants, suggest that the vector is capable of accumulating a significant amount of maize streak virus particles.

Leafhoppers acquire the virus through their food canal during feeding from a plant cell, from which the virus then move towards the midgut and spread through the insect. The minimum acquisition time is less than 15 seconds and the virus may be transmitted to host plants in less than five minutes.

Some members of the leafhoppers, such as C. mbila, are capable of transmitting the virus immediately after feeding on the infected plants or after it has been injected into the haemolymph. However, there are some species of leafhoppers, such as C. chinaï, where transmission takes place only after the virus has passed through the haemolymph.

Maize streak virus is located in the mesophyll tissue of infected maize and initial contact may occur when insects feed on the mesophyll cells. Leafhoppers feed on the phloem sap inside the plant, releasing the virus to the phloem sieve tube of the host, where it will then replicate, thus increasing the chances of infection.

Control

Maize streak virus is difficult to control and as a result, farmers suffer high yield losses when infections are severe, particularly subsistence farmers. Planting maize in an open area where there are no trees could reduce the number of infections, as leafhoppers prefer shade.

Weeds harbour insects and diseases that could results in infection spreading to the crop. Leafhoppers that transmit maize streak virus survive on several graminaceous weeds species as alternative hosts, therefore intensive control of weeds can minimise maize streak virus incidences.

Intercropping can also reduce the number of infections, as this might lead to confusion for the vectors in finding the maize plant for feeding. Use of barriers such as bare ground between early and late-planted maize to limit movement of leafhoppers could also reduce the spread of the virus. Maize hybrids that are resistant to maize streak virus play an important role in reducing maize streak virus incidence in integrated pest management systems.

Although insecticides, such as Gaucho® and Cruiser®, are available for the control of maize streak virus, most subsistence farmers do not use them for disease control because of their high cost and potential health risks.

The complexity of maize streak virus epidemiology makes control achievable by implementing an integrated pest control strategy. Farmers are therefore encouraged to cultivate treated seed of improved hybrids every season in order to obtain good yield production.

For more information on sunflower white rust and grey stem spot, contact the authors at (018) 299-6100 or FlettBC@arc.agric.za and RamusiM@arc.agric.za.