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Root yield, propagation material yield, and growth responses to Mononychellus mite (Acari:Tetranychidae) populations, and foliage damage ratings of resistant and susceptible cultivars (previously determined by foliage damage ratings) were compared in protected and unprotected plots to confirm the level of resistance, to relate foliage damage estimates with economic yield reductions, and to study the general effectof mite attacks on cassava. On the average, cassava cultivars selected as susceptible had larger reductions in leaf size, leaf life, leaf formation rate, plant height, root yield, yield of propagation material, root weight, root number, and percent marketable roots than did the resistant cultivars when compared to their respective performance in protected plots. The average yield loss for susceptible cultivars was 73% for roots and 67% for propagation material; the respective losses for the resistant cultivars were 15 and 19%. Our selection criterion for resistance (i.e., foliage damage ratings) was correlated with mite density and associated with the percent yield loss caused by the mites indicating this criterion's potential value in future breeding efforts, although exceptions exist due to the presence of tolerance.

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