First Report of Powdery Mildew of Tomato Caused by an Erysiphe sp. in Connecticut

A new spore precipitator with polarized dielectric insulators for physical control of tomato powdery mildew

ABSTRACT In an attempt to physically protect greenhouse tomato plants from the powdery mildew fungus Oidium neolycopersici, we developed a new electrostatic spore precipitator in which a copper wire conductor is linked to an electrostatic generator and covered with a transparent acrylic cylinder (insulator). The conductor was negatively charged by the generator, and the electrostatic field created by the conductor was used to dielectrically polarize the insulator cylinder. The dielectrically polarized cylinder also produced an electrostatic force without a spark discharge. This force was directly proportional to the potential applied to the conductor and was used to attract conidia of the pathogen. The efficacy of this spore precipitator in protecting hydroponically cultured tomato plants from powdery mildew was evaluated in the greenhouse. The hydroponic culture troughs were covered with a cubic frame installed with the spore precipitator, and the disease progress on precipitator-guarded and unguarded seedlings was traced after the conidia were disseminated mechanically from inoculum on tomato plants. Seedlings in the guarded troughs remained uninfected during the entire experiment, in spite of rapid spread of the disease to all leaves of the unguarded seedlings.

Formation of Conidial Pseudochains by Tomato Powdery Mildew Oidium neolycopersici

The formation of conidial pseudochains by the tomato powdery mildew Oidium neolycopersici on tomato leaves was monitored using a high-fidelity digital microscope. Individual living conidiophores that formed mature conidial cells at their apex were selected for observation. The conidial cells were produced during repeated division and elongation by the generative cells of the conidiophores. Under weak wind conditions (0.1 m/s), these conidial cells did not separate from each other to produce a chain of conidial cells (pseudochain). The pseudochains dropped from the conidiophores once four conidial cells were connected. The conidiophores resumed conidium production, followed by another cycle of pseudochain formation. The formation of pseudochains by tomato powdery mildew was not influenced by the ambient relative humidity. On the other hand, the conidial cells produced were easily wind dispersed without forming pseudochains when conidiophores were exposed to stronger winds (1.0 m/s). The present study successfully demonstrated that the pathogen required wind to disperse progeny conidia from the conidiophores and produced conidial pseudochains when the wind was below a critical level, independent of high relative humidity as reported previously.

Molecular Identification of Oidium neolycopersici as the Causal Agent of the Recent Tomato Powdery Mildew Epidemics in North America

A previous morphological study of Oidium anamorphs responsible for the recent tomato (Lycopersicon esculentum) powdery mildew outbreaks worldwide suggested that, despite controversial data in the literature, the North American epidemics were caused solely by a newly erected species, O. neolycopersici. We report here the first molecular evidence that the North American anamorphs do belong to O. neolycopersici. The internal transcribed spacer sequences of the North American anamorphs of this study were identical with those of three Japanese and four European specimens of O. neolycopersici. A morphological study confirmed that all the North American Oidium anamorphs included in this study produced conidia singly, similar to O. neolycopersici. These fungi were readily distinguished from O. lycopersici, which produces conidia in chains and is known to infect tomato only in Australia. The phylogenetic analysis showed that O. neolycopersici is a distinct powdery mildew species, and it is neither identical nor closely related to any known polyphagous species of the Erysiphaceae. Apparently, it was introduced into the United States and Canada only in the 1990s, but its origin is still unknown.

The tomato powdery mildew fungus Oidium neolycopersici

summary Pathogen: Powdery mildew fungus; Ascomycete although sexual stage is yet to be found; an obligate biotroph.

IDENTIFICATION

Superficial mycelium with hyaline hyphae; unbranched erect conidiophores; conidia, ellipsoid-ovoid or doliform, 22-46 x 10-20 microm, lack fibrosin bodies; conidia formed singly, rarely in short chains of 2-6 conidia; appressoria lobed to multilobed, rarely nipple-shaped. Pseudoidium species.

HOST RANGE

Broad, reported to attack over 60 species in 13 plant families, particularly members of the Solanaceae and Curcubitaceae.

SYMPTOMS

Powdery white lesions on all aerial plant parts except the fruit. In severe outbreaks the lesions coalesce and disease is debilitating. Agronomic importance: Extremely common in glasshouse tomatoes world wide but increasing in importance on field grown tomato crops.

CONTROL

Chemical control and breeding programmes for disease resistance.