Blossom Blight on Macadamia

Maritha Schoeman Plant disease specialist ARC-TSC

The summer rains came early this season but the rain also brought a reason for great concern for the macadamia growers in the form of possible flower diseases.  Flowering this season was extremely good, but the rain came at a time when many flowers were only a few days (5-10 days) past the full bloom stage and very susceptible to blossom blight.  Some flowers were fortunately further developed with many flower remnants falling from the racemes just as the ovaries began to swell. These flowers are not susceptible.

In South Africa blossom blight is caused by the fungus Cladosporium cladosporioides. Symptoms: Diseased racemes are covered with olive grey patches of mycelia and abundant spores.  The fungus is present every year and the significance of the disease is dependent on weather conditions prevailing during the susceptible flower stages. Orchard epidemics can occur when rainy weather prevails for three or more days and temperatures range from 20 to 25°C during this period.  As these weather conditions were experienced the past few weeks severe blossom blight epidemics occurred in many areas, sometimes irrespective of preventative sprays applied.

In trials the ARC-TSC conducted under the leadership of Maritha Schoeman, the effect of spraying were compared with non-spraying in terms of fungal presence and nut set. A total of 1920 racemes were monitored during the season at six production sites, on cultivars 816 and Beaumont. Sprays were applied by the growers when flowers were fully extended and have just started to open and again three weeks later. At the first rating it was evident that spraying did reduce the presence of fungal growth on the racemes from 75% to 0% at five of the producers. The reason for the lack of reduction at the one producer was because the fungicide was applied as a light cover spray compared to a full cover spray at all the other producers. Results clearly showed that fungicides had an effect on the fungus. At the second rating of the disease, when nuts were ± 20 mm in diameter and withered flower parts and unset nuts had already fallen, fungal incidence decreased significantly even in the unsprayed blocks (20% vs 75% at first rating). The reason for this is that there were little dead or dying tissue left for the fungus to live on.

There was a fair amount of fungus present in the unsprayed trees, and zero in the sprayed trees but there was not a huge difference between sprayed and unsprayed blocks in terms of nut set. These results hinted that the fungus is maybe of little importance, but this needs to be confirmed. There is still a lack of adequate knowledge about the disease and its influence on nut set in particular.

Spraying should only be done preventatively if there is a moderate to severe disease risk for flower blight and the aim is to apply fungicides so that it systemically penetrate flowers before they enter the stage were flowers have been open for 5 to 10 days. If a disease epidemic occurs and a large portion of the flowers are in the stage where all flowers on the raceme have already been opened 5-10 days, it is too late to begin spraying. During dry seasons disease control are not warranted at all.

Sprays should be considered on a site by site basis and at most sites, intervention will in most seasons not be required.  Pruning or thinning out of trees is extremely important allowing more sunlight into the trees, thus rendering conditions inside the orchard unfavorable for disease proliferation, at the same time promoting flowering and nut set.

  • In Australia, Botrytis cinerea is the main cause of Blossom blight and Cladosporium to a much lesser extent. Researchers there also found that due to a lack of adequate knowledge about these diseases, attempts to control raceme blights with fungicides during severe outbreaks have mostly not been successful. They also found an absence of Botrytis blight during disease conducive periods when heavy rain actually stripped all the dead flower parts from the raceme leaving no dead or dying tissue for the fungus to live on.

With the first heavy rain to follow from now on, dead tissue will also be stripped from the racemes leaving no dead tissue for the fungus to live on. The application of any kind of fungicide at this stage is not recommended as it is deemed too late.

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