Plant Disease of the Week No. 12c (Sooty mold)

Disease name: sooty mold.

Definition: a black, nonparasitic, superficial fungal growth on honeydew produced by aphids and other phloem-feeding insects (American Phytopathological Association, www.apsnet.org).

Worldwide significance: Most sooty molds warrant no management because they pose not major threat to plant health. Sooty mold management is warranted where the molds affect the aesthetic value of fruits and landscapes.

Hosts: Any plant that is a host to phloem-feeding insects is susceptible to sooty mold, or an host upon which honeydew is deposited by phloem-feeding insects.

Geographic range of sooty molds: widely distributed throughout temperate and tropical regions globally.

Fungi: Some of the common genera of fungi found in sooty mold complexes are Cladosporium, Aureobasidium, Antennariella, Limacinula, Scorias, and Capnodium.

Site of infestation: foliage

Host range of the associated fungi: the fungi are indiscriminate

Associated insects: sap- or phloem-feeding insects such as whitefies, aphids, mealybugs, and scales.

Disease cycle:

Dissemination - spores of the causal fungi are borne on air currents or by rainwash or rainsplash.
Inoculation - spores land on a leaf surface (or other foliar aspects) upon which a sheen of honeydew has been deposited by phloem-feeding insects.
Infection and pathogen development - no infection ever occurs; sooty mold fungi are not plant pathogens.
Symptom and disease development - using the sugary honeydew as a food source, the sooty mold fungi begin to grow on the surface of plant foliage, turning the foliage various shades of black.
Pathogen reproduction - more fungal spores are produced on foliage.
Pathogen survival - the fungi survive saprophytically as mycelium or spores on plant debris or on inanimate human-made objects such as vehicles.

Above: Sooty mold cycle. In this series of events, the phloem-feeding insects often are tended and protected by ants, who feed upon the excreted honeydew and protect the phloem-feeding insects from their natural enemies (Diagram - S. Nelson, UH-CTAHR, 2008).

Conditions favoring the development of sooty mold: sooty mold forms under a wide range of environmental conditions in Hawaii. Some sooty molds are favored by cool, moist, humid conditions.

Negative effects of sooty mold upon plants: (1) reduced photosynthesis and gas exchange; (2) cosmetic damage to marketable fruits and damage to the pleasing aesthetic attributes of landscapes; (3) increased mold counts in processed juices and purees made from infested materials.

Types of sooty molds: Sooty mold growth is of two types. The first is a deciduous growth on leaves, which lasts for the life of the leaf. The second is persistent growth on stems and twigs of woody plants and on human-made structures or objects. In this type, growth is renewed from existing mycelium of the fungi produced the previous season.

Rule of thumb: if you can rub the black material off of the leaves or plant surfaces with your fingers, it is probably only sooty mold, if it remains attached to the plant and you cannot rub it off, it is probably not sooty mold.

Sooty blotch. Sooty blotch diseases are similar to sooty molds, but are not associated with phloem-feeding insects. In other words, their formation and growth does not depend upon honeydew. They can occur, for example, on banana fruits and on petioles of palms in Hawaii.

Sooty mold on outdoor, non-plant structures: On outdoor structures and furniture, sooty mold growths are unsightly and may be difficult to remove.

Allergenic properties of sooty mold fungi: Many people are allergic to sooty molds, particularly species in the genera Cladosporium and Aureobasidium.

Integrated management of sooty molds:

The best way to control most sooty molds is to prevent them from becoming established. The best way to prevent sooty mold from becoming established on plant foliage is to control phloem-feeding insects on the foliage. Controlling the phloem-feeding insects may require additional control of the ants that tend and protect them.

Once sooty mold is established, an insect control program should be started. Choice of insecticide depends upon the site of application and the target pest(s).

If high-value plants are affected, sooty mold can be carefully washed from plant leaves usign soapy water.

And, because sooty molds are fungi, general-purpose fungicides applied to control other diseases may have some effect in killing sooty mold fungi, although the fungicides will not remove the black color from the leaves.