Left: Grape cluster infected with powdery mildew.
Center:Light micrograph of a pycnidium of A. quisqualis which has formed in place of a conidium atop the conidiophore of Uncinula necator. Conidia of A. quisqualis have been released from a rupture of the pycnidial wall.
Right: Electron micrograph of powdery mildew colony on grape leaf showing pycnidium of A. quisqualis in conidiophor of U. necator. D.Gadoury (all photographs).

Ampelomyces quisqualis
(Deuteromycetes)

by David Gadoury, Department of Plant Pathology, Cornell University-NYSAES, Geneva NY

The fungus Ampelomyces quisqualis is a naturally occurring hyperparasite of powdery mildews. It infects and forms pycnidia (fruiting bodies) within powdery mildew hyphae, conidiophores (specialized spore-producing hyphae), and cleistothecia (the closed fruiting bodies of powdery mildews). This parasitism reduces growth and may eventually kill the mildew colony. A. quisqualis has been the subject of numerous investigations on biological control of powdery mildews for over 50 years.

Appearance

Parasitized mildew colonies are dull, flattened, and off-white to gray in color. Spore production of the powdery mildew is reduced or absent in parasitized areas of the colony. Dead leaf tissue may be adjacent to parasitized areas of the mildew colony, as the outer leaf tissues die in response to death of the contained powdery mildew cells.

The pycnidia of A. quisqualis vary in shape depending upon the organ in which they form. Within conidiophores they are pear-shaped, within hyphae they are spindle-shaped, and within cleistothecia they may be nearly spherical.

Uninfected hyphae and conidiophores of powdery mildews are transparent, but turn translucent-white soon after infection. Once the mycoparasite has begun to produce pycnidia, the hyphae and conidiophores swell to several times their normal diameter, and the amber color of the pycnidial wall of the mycoparasite may be noticed through the cell walls of the host.

Parasitized cleistothecia are typically dull, fawn colored, flaccid, and range in size from 64 to 130 micrometers in diameter. Pycnidia contain cylindrical to spindle-shaped, occasionally curved, and two-spotted conidia that are 7.5-9.0 X 2.5-3.5 micrometers. The conidia may be exuded when parasitized tissues are exposed to free water or humidity near saturation.

Habitat (Crops)

A. quisqualis exists in colonies wherever its many hosts are distributed.

Pests Attacked

The mycoparasite is specific to powdery mildews (Erysiphales), but has an extremely broad host range within this diverse group of important plant pathogens. It has been recorded on more than 64 species in the genera Brasilomyces, Erysiphe, Leveillula, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, and Uncinula, as well as the anamorphic genera Oï dium and Oï diopsis. These reports represent powdery mildews which attack 256 plant species within 172 genera in 59 families, and which occur in 28 countries around the world.

Life Cycle

The pathogen overwinters or survives intercrop periods as pycnidia. The relative importance of pycnidia within different host structures (i.e., hyphae vs conidiophores vs cleistothecia) as survival structures is unknown. When it rains, conidia are exuded from the ripe pycnidia and are dispersed by rain-splash to mildew colonies. Free water is required for infection, and infection is favored by warm temperatures (20-30° C). Under favorable conditions, infection can occur in less than 24 hours.

The mycoparasite directly penetrates the walls of hyphae, conidiophores, and immature cleistothecia, but may be unable to infect mature cleistothecia. For approximately 7-10 days, the mycoparasite spreads within the hyphae of the mildew colony without killing it. Thereafter, the process of pycnidial formation begins, and is then completed within 2-4 days. Infected cells generally die soon after pycnidial formation begins. Secondary cycles of infection result from conidia released during subsequent rain events.

Several generations may occur during a single growing season.

Relative effectiveness

Efficacy in controlling powdery mildews has been quite variable. The fact that some powdery mildew must be present to serve as a base of establishment for the mycoparasite makes this a less than ideal means to control powdery mildews on crops with no tolerance for disease (either extremely high susceptibility, high value, no tolerance for defects, or all of the foregoing). However, a number of examples of acceptable disease control have been reported for greenhouse and field-grown vegetable crops. Repeated applications of the mycoparasite are generally necessary, and high humidity and rainfall aid in spread to developing mildew colonies.

Pesticide susceptibility

Isolates differ in their susceptibility to fungicides. However, isolates tolerant to many of the most commonly used fungicides have been collected and are available for use. Susceptibility to other pesticides is poorly understood.

Conservation

A. quisqualis may survive in parasitized cleistothecia on the bark of deciduous perennial hosts, as well as on colonies on fallen leaves and crop debris. However, reintroduction on an annual basis is likely to be necessary for commercially acceptable disease control in annual or greenhouse crops.

Commercial Availability

A formulated powder containing A. quisqualis is available from Ecogen Corporation and is marketed under the trade name AQ10.

References

Falk, S.P., Gadoury, D.M., Pearson, R.C., and Seem. R.C. 1995. Partial control of grape powdery mildew by the mycoparasite Ampelomyces quisqualis. Plant Dis. 79:483-490.

Falk, S.P., Gadoury, D.M., Cortesi, P., Pearson, R.C., and Seem, R.C. 1995. Parasitism of Uncinula necator cleistothecia by the mycoparasite Ampelomyces quisqualis. Phytopathology 85:794-800.

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