Choosing the right fungicide for citrus crop is an essential decision that cultivators have to make for ensuring a healthy harvest
Centuries of citrus breeding outside the natural habitat of the crop, has created a broad genetic base of commercially grown citrus equipped to produce a higher yield across a wider range of growing conditions. Cultivars now grow citrus in Equatorial climates with its year-round high temperature, high humidity and rainfall and also classic Mediterranean type of climates with hot dry summers and cool wet winters and some sub-zero temperature.
Net result is a wide range of citrus genotypes grown in a large number of countries, climate and environment resulting in specific fungal and bacterial pathogens evolving on particular types of citrus in different climatic and agronomic situations.
Fungicide requirements
Citrus growers, who are faced with a wide range of plant diseases, clearly need a ‘one fits all’ fungicide to make spray application programmes as cost effective as possible. Broad spectrum fungicide activity from weatherproof spray deposits, freedom from fungicide resistance and no phytotoxic citrus fruit reaction (called ‘stippling’ or blemishing) are all essential requirements.
Particulate fixed copper compounds which furnish fungicidally active Cu2+ ions are the only products with this all-round ability. These sparingly soluble salts of metallic copper, which include copper oxychloride, cupric hydroxide and cuprous oxide, are universally sprayed to control citrus diseases.
Spray deposits of particulate fixed copper fungicides like cuprous oxide show greater adhesion than cohesion because the particles stick to plant surfaces with greater force than they stick to each other. The smaller the particles, the more uniform is the spray coverage and the greater the chance of particles landing and adhering to the actual plant surface rather than another particle of copper fungicide. The larger surface area (relative to mass) of smaller particles means the spray deposit has more contact with the plant surface and greater adhesive forces to enhance spray deposit retention and resistance to weathering.
Main factors affecting spray deposit retention of copper fungicide deposits on citrus leaf and fruit surfaces are:
• Rains: Dislodgement of particles by the physical force of falling raindrops and loss of fungicide during solubilisation from washing and run-off effects of rainfall and/or condensation from the atmosphere as dew;
• Wind events: Particles over three to four µm in diameter are blown off plant surfaces due to their weaker adhesive forces. Abrasive effects of leaves and the fruit rubbing together during wind-caused movement can dislodge particles.
• Physical removal of particles on plant surfaces due to plant growth movements with tension in leaves and fruit.
Loss of spray deposit by weathering is generally exponential with initial high losses caused by dislodgement and removal of larger particles, leaving a residue of increasingly smaller particles. Reports suggesting that single large dose applications are less effective than the same amount of fungicide split between two or more repeat applications may be due to lower cohesive forces created when particles are deposited on top of each other rather than directly onto the leaf surface. In other words a particle of copper fungicide in contact with other particles is more easily dislodged than if in direct contact with the actual leaf surface.
Copper fungicides in action on citrus
Copper fungicides are purely protectant in action. Disease control relies on maintaining an even distribution of spray deposit with good retention over all susceptible citrus plant surfaces. By definition, protectant copper sprays must be applied evenly over the leaf or fruit surface and before spore germination and infective penetration by the germ tube can occur. The superiority of fixed copper compounds (such as cuprous oxide) as protectant fungicides is its ability to stick firmly on the plant surface where it lands. From this initial deposit position, copper ions are gradually released by solubilisation to enter the germinating fungal spore and kill them before they can infect the leaf or fruit.
By Terry Mabbett
To continue reading the rest of this article, please see the June/July 2013 issue of Far Eastern Agriculture
