ThePhytophthora infestanszoospore contains five out of six rootlets that are found in other secondary-type oomycetous zoospores. They include (i) an anterior ribbed rootlet (AR4) that runs along the right side of the anterior groove and consists of four main microtubules, an electron-opaque cord, and secondary (rib) microtubules; (ii) an anterior rootlet (A2) that runs on the left side of the anterior groove and consists of two microtubules; (iii) a posterior rootlet (P6) that runs on the left side of the posterior groove and consists of six microtubules; (iv) an array of cytoplasmic microtubules; and (v) an array of nucleus-associated microtubules. There is no multistranded, band-shaped rootlet (MS). There is a concertina-like structure in the core of the transition zone distal to the basal plate. Differences in rootlet morphology may provide a useful means of classifying species ofPhytophthora.Key words: flagellar apparatus, zoospore,Phytophthora infestans, Oomycetes.
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