Black point
Black point is a dark discolouration at the germ end of otherwise healthy wheat grain. Winter cereals, particularly those grown outside a classic Mediterranean climate, often encounter harmful weathering conditions during grain development.
Winter crops in the subtropics often ripen with high humidity or rain. High humidity alone can stall or prolong ripening, initiate germination, induce excessive enzyme activity and establish colonisation by saprophytic fungi and bacteria. In wheat, high humidity induces a clean-edged symmetrical-melanized pattern in the pericarp and testa cells covering the germ and adjacent areas.
Black point does not affect grain health or germination. It has been found that black pointed wheat grain actually germinates faster than normal grain and is consequently predisposed to sprouting should further rainfall occur.
However, in combination with shrivelling, field mould or disease, grain health may be affected and germination rates reduced.
Black point occurs more often in wheat grain of larger size and high-yielding wheat crops are generally worse affected.
In areas where black point is a recurring problem, farmers should avoid highly susceptible varieties. Growing more than one variety with multiple planting times will reduce losses if black point occurs.
Although various fungi are associated with black point, there is no conclusive evidence that black point melanism is a direct result of fungal action. The often reported association between symptoms and the most common grain coloniser, Alternaria alternata, is not strong and several studies have been unable to confirm any link.
Black point first appears when the grain begins to lose moisture. Prolonging or reversing the drying process allows interaction between enzymes and substrates that normally lie dormant until subsequent germination. Black point does not occur after grain maturity.
Melanism
Melanism (sometimes referred to as false black chaff) is not a disease. It is an over expression of melanoid pigments in the plant causing a dark-brown to black discolouration of heads and exposed stems below the head. It is associated with the Sr2 gene, which is used as one of the primary sources of stem rust resistance in a number of our current varieties.
The degree of discolouration is controlled by several modifier genes and is strongly influenced by environmental conditions. Melanism is induced by prolonged wet weather, high humidity and high levels of UV radiation. It was very conspicuous in the 1998 season, particularly in the variety Pelsart.
