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 spacing (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 identification as the photos are not good enough. 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 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 G citricarpa which is found in every place that has been surveyed for diseases; again, generally not economically damaging. Two photos are attached (lower row). It is often referred to as “shot hole” because the middle drops out as the spots age.
Sometimes the spots of Cephaleuros overgrow those of Guignardia citricarpa; the former can be seen as “rhizoids” (root-like) patterns growing over the spot.
Anthracnose (Colletotrichum gloeosporioides/Glomerella cingulata) has also been recorded from noni, but less often than the other two.
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 grow the trees using traditional practices in order to prevent diseases that are not normally economically important from becoming a problem.