Knowledge of diseases is pivotal for making sound decisions

December 2019

Magda du Toit, Corporate Engagement and Communications Manager SA, Bayer. Send an email to magda.dutoit@bayer.com

Cool soil temperatures and poor drainage creates favourable conditions for the development of seed rots as well as root and seedling diseases. Often, in a field, areas with poor germination/emergence and vigour should be noted and monitored at seedling emergence and stand establishment, and where possible identify the cause of the problem.

Root diseases are caused by a variety of soil pathogens that include species of Pythium, Fusarium and Rhizoctonia as well as nematodes. Pythium species require excessive moisture (for their motile zoospores to swim) and are likely to be a problem in soils with poor drainage. Stressed plants, due to extreme temperature and/or moisture, herbicide damage and physical damage (by insects, strong winds or machinery) are more prone to infection by Fusarium and Rhizoctonia species.

Certified seed is almost always sold already treated with fungicides, insecticides and sometimes nematicides. The seed treatment only provides protection for a few weeks after planting and can be overwhelmed in conditions of high disease pressure and when conditions are not favourable for rapid emergence and plant growth. Low temperatures delay germination and emergence and lowers early seedling vigour. 

Crop residue left on the soil surface insulates the soil from solar radiation which slows down the warming of the soil. The slow germination and emergence provide more time for the soil-borne pathogens to attack the seed or seedlings and infect roots. Stalk rots probably also begin as root and crown rots early in the season that manifest later in the growing season. Incorporating residue into soil will reduce insulation provided by the residue in the soil but will not significantly reduce inoculum potential because soil-borne pathogens are well equipped for life in the soil. Strip-till methods move residue to the side and leave a narrow band in the planted row, should reduce soil insulation from soil radiation (increase soil solarisation) directly where the seed is planted.

Seed and seedling diseases can be minimised by planting high quality seed in well drained soils, at appropriate seed depth. Fields with a history of high prevalence of seedling diseases can benefit from in-furrow fungicide and insecticide application at planting. In the case of systemic insecticides protection against sucking pests such as leafhoppers that transmit Maize Streak Virus only lasts for between 3 to 6 weeks and under high insect/vector pressure follow-up insecticide sprays may be necessary. 

FOLIAR DISEASES
Good rains can result in a high prevalence of foliar diseases such as Northern leaf blight, grey leaf spot, Phaeosphaeria leaf spot (white spot) and common rust. Foliar diseases can have a significant yield reduction effect and also affect stalk health resulting in poor standability late in the season. Most of these above ground diseases are caused by pathogens that depend on the host residues for survival between seasons and the initial inoculum for the next season.

In the case of conservation tillage these pathogens are on the surface and already close to the plant where they increase disease potential. The burying or incorporation of the plant residues into the soil reduces inoculum potential in two ways. First, the inoculum is no-longer in close proximity to the host plant and second, by the decomposition of the plant residue. Buried plant residues remains continuously moist and is colonized by saprophytic soil microorganisms (bacterial and fungi) that decompose the plant residue. The decomposition of the residue deprives the pathogens of nutrients and some of the soil microorganisms are antagonistic to the pathogens. Most of the leaf foliar pathogens cannot survive in residue under the soil for more than a few months. 

The history of the field as regards the previous crop and the prevalent diseases in the previous season should inform the producer on how early to incorporate the residues to reduce pathogen inoculum. 

Diseases that appear in one season are likely to carry over to the next season. Knowledge of the exact disease occurring, its prevalence and severity is pivotal for making sound management decisions for the next season’s crop.  

Publication: December 2019

Section: Pula/Imvula