October 2005. Leaf spots of noni, Morinda citrifolia, Samoa (top row). The leaf spots appear to have increased in severity in recent weeks, and are now of economic significance, causing major damage to the trees. It is estimated that 60-80 leaf fall has occurred with death of branches. The spots occur on all but the youngest leaves. The noni are grown in a plantation at 3 metre spaciing (538 trees per ha).

It was generally considered difficult to identiy leaf spot diseases from photographs, and If a proper diagnosis is desired, specimens should be sent for taxonomic identifiction as the photos are not good enough for a proper diagnosis. SPC provided a situation report on diseases of noni in Pacific Island countries.

Two main leaf problems are seen frequently in the Pacific on noni.

1. Cephaleuros spp.(lower, left). Normally not an issue and under most conditions is not economically damaging. It is often the result of shade and high humidity and is found on many trees including mango, avocado, etc. It can be fairly easily controlled if that is the desire of the grower by opening up the canopy, improving host nutrition and generally practicing good husbandry. For a very useful and interesting paper on Cephaleuros, see Fred Brooks, American Samoa Community College, pest information sheet. http://www2.ctahr.hawaii.edu/adap2/ascc_landgrant/Dr_Brooks/TechRepNo39.

2. Guignardia citricarpa (lower, right). The most common leaf spot of Morinda in the Pacific is Guignardia citricarpa which is found in every place that has been surveyed for disease; again, generally not economically damaging. Two photos are attached (lower row). It is often referred to as “shot hole” because the middle drops out of the older spots.

Sometimes the spots of Cephaleuros overgrow those of Guignardia citricarpa. The difference is the “rhizoid” (root-like) patterns you can see growing over the spot.

Anthracnose (Colletotrichum gloeosporioides/Glomerella cingulata) has also been recorded from noni, but to a much lesser frequency than the other two organisms.

There is a need to consider the implications of turning noni into a commercial crop and growing it in “orchards”. Hawaii is having problems with a Phytophthora and the Cook Islands (as mentioned recently on PestNet) is suffering from a serious infestation of root-knot nematode because of orchard-style management. In order to avoid creating disease situations, it might be best to grown the trees using traditional growing practices in order prevent diseases that are not normally economic from becoming a problem.