Interaction of spatially separated Pratylenchus penetrans and Verticillium dahliae on potato measured by impaired photosynthesis

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Abstract
Experiments were conducted under growth‐chamber conditions to determine if Pratylenchus penetrans systemically alters light use efficiency (LUE) of Russet Burbank potato infected by Verticillium dahliae. Pathogen separation was achieved by inoculating potato roots with the nematode prior to injecting fungal conidia into the stem vasculature. Treatments were P. penetrans alone, V. dahliae alone, nematode and fungus together, and a no‐pathogen control. Gas exchange was repeatedly and nondestructively measured on the fifth‐youngest leaf with a Li‐Cor LI‐6200 portable photosynthesis system. By 16 and 20 days after stem injection with the fungus, LUE was synergistically impaired in jointly infected plants. Transpiration in plants infected with both pathogens was significantly reduced. However, the combined effect of nematode and fungus was synergistic in one experiment and additive in the other. Stems were destructively harvested when LUE was synergistically impaired. Coinfected potato plants contained more colony‐forming units (CFU) of V. dahliae in stem sap than those infected by the fungus alone in one experiment. Evidence is provided that infection of Russet Burbank roots by P. penetrans systemically affects disease physiology associated with stem vascular infection by V. dahliae . The findings indicate that the role of the nematode in the fungus/host interaction is more than simply to facilitate extravascular and/or vascular entry of the fungus into potato roots.

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