Fusarium graminearum species complex on maize has reached epidemic proportions

SONIA-MARI GREYLING AND BRADLEY FLETT, ARC-GRAIN CROPS INSTITUTE, POTCHEFSTROOM

Fusarium graminearum (also known as Gibberella zeae) is the causal agent of Gibberella ear rot, seedling blight, root and stalk rot of maize and Fusarium head blight of small grain crops.

A large number of cereals are affected by this pathogen, including wheat, barley, rye, sorghum and maize. It often happens that planting cereals in rotation increases the risk of infection. Other factors contributing to infection are optimum climatic conditions and susceptible cultivars.

Prior to 2000, it was thought that F. graminearum (Gibberella zeae) was a single species found worldwide. Recent research has found that the complex of fungi classified as F. graminearum had host, climatic and regional preferences. Through genetic analyses these have been divided into eight biogeographically distinct lineages, which became known as the F. graminearum species complex (FGSC).

This was later revised to 13 species. These are F. Austroamericanum (lineage 1), F. meridionale (lineage 2),
F. boothii (lineage 3), F. mesoamericanum (lineage 4), F. acacia-mearnsii (lineage 5), F. asiaticum (lineage 6),
F. graminearum sensu stricto (lineage 7), F. cortaderiae (lineage 8), F. brasilicum (no lineage number),
F. aethiopicum (no lineage number), F. gerlachii (no lineage number), F. vorosii (no lineage number), and
F. ussurianum (no lineage number).This is why we now refer to the pathogen complex as the F. Graminearum species complex (FGSC) rather than the previous name of F. graminearum or G. zeae.

Five species of the complex have been shown to be associated with maize. These are F. asiaticum,
F. austroamericanum, F. boothii, F. meridionale, and F. graminearum sensu stricto. It has been shown that the species differ in the degree to which they cause disease (virulence) and tissue specificity. In South Africa, these five species were isolated and identified from maize roots and crowns. However, in maize ears, only F. boothii has been recorded. Of these species, F. boothii are the most virulent and F. meridionale the least.

Symptoms

Gibberella ear rot of maize can be identified by dark pink/red mycelium growth spreading from the silks downwards (Photo 1). Early infection of physiologically mature ears can result in mycelia tightening the ear leaf sheath around the ears.

Symptoms of Fusarium crown and stalk rot include early death of the plant and pink to reddish mycelium growth on the pith (Photo 2). As the fungus ramifies, the pith pulls away from the outer rind, of the stalk. This results in hollow tubes that, in some cases, are unable to support the plant which results in maize lodging.

Fusarium root rot symptoms can range from very slight brownish to dark-black discoloration. Infected roots can also be red or pink. During advanced stages of infection the roots can become completely rotted (Photo 3).

Mycotoxins

Mycotoxins are secondary metabolites produced by fungi, which causes mycotoxicosis in humans and animals. The FGSC produces three different types of mycotoxins, namely Deoxynivalenol (DON), Nivalenol (NIV) and Zearalenone (ZEA).

These secondary metabolites are often advantageous to the pathogen. For instance, DON allows the pathogen to colonise its host easier. Unfortunately these mycotoxins have negative effects on human and animal health.

In humans DON and NIV can cause symptoms such as nausea, fever, headaches, and vomiting. ZEA has an estrogenic effect and can cause an enlarged uterus, swelling of the vulva and vagina (known as vulvovaginitis), enlarged mammary glands, anestrus (periods of infertility), and abortion in animals. Swine are particularly vulnerable, with cattle and poultry less affected.

The perception used to be that mycotoxins are only found in tissue that was infected with the fungus. However, recently it was shown that DON can be transported from stalk tissue to the grain within wheat plants. The ARC-GCI is studying this phenomenon in the maize – F. graminearum pathosystem. If this proves to be the case, it is a serious disadvantage as infected tissue can’t be separated from healthy tissue anymore in order to conserve quality, because the healthy tissue may be contaminated with mycotoxins.

Life cycle and epidemiology

A major source of FGSC inoculum comes from previous year’s plant material. Spores are formed in special structures on the residual plant material on and in the soil. In the new growing season, spores are disseminated and infect the plant either through the silks, the leaf sheath or the roots. The pathogen establishes itself inside the plant before it starts killing tissue. At which point symptoms become visible.

Cultivar susceptibility hinges on a range of factors, such as the environment and the plant self. The growth stage of the plant where fungal attack by the pathogen is most prevalent is 21 days after silking.

Environmental factors play an important role as well, as F. Graminearum prefers warm weather of 25°C to 28°C with a high humidity.Abiotic stresses, such as water logging, aggravate the disease.

Control measures

Unfortunately there are no resistant cultivars available; only those that are more or less tolerant to infection. It has been shown that plants under stress will use all its resources for grain fill, which usually results in stalks with fewer nutrients such as carbohydrates. This predisposes the plant to pathogen infection. Lowering the plant’s stress level, in combination with a tolerant cultivar, reduces the possibility and severity of infections.

Crop rotation is important as it has been found that planting cereals in succession results in higher disease incidence. Rotating with a nonhost crop reduces inoculum build-up. This is especially important in fields where no or limited till practices are applied. In Europe the planting and ploughing in of mustard has been shown to reduce pathogen survival. This process is commonly called bio-fumigation.

Another control measure is to remove the previous season’s crop residues. This reduces the available plant material that the pathogen can use to overwinter. Less material means less primary inoculum available for possible infection. However, infection only occurs when the growth conditions are optimal for the pathogen.

This control measure must be incorporated with proper crop rotations. FGSC species have been found to survive saprophytically on non-graminaceous residue as well. This may explain the continued high incidence of FGSC related stalk rots in fields that have been rotated to non-graminaceous crops. Further research studies on the survival and spread of FGSC are being done by the ARC-GCI.

Plant pathologist may be contacted at the ARC-GCI at 018 299 6100, for further information or for assistance in developing management strategies.