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Disease variation of Sphaceloma manihoticola isolates affecting cassava in Barbados.

Angela Alleyne: The University of the West Indies, Cave Hill Campus

<div><em>Sphaceloma manihoticola,</em> (Bitancourt & Jenkins) causal agent of Superelongation disease (SED) of cassava <em>Manihot esculenta</em> (Crantz) is an important pathogen in the Caribbean, given its potential to negatively impact the cassava industry. It produces increased quantities of gibberellin GA<sub>4 </sub>in late stages of infection by over-expression of <em>Smp450 </em>genes <em>in-planta. </em>This study sought to characterize SED in varying rainfall zones in Barbados.</p> <p>From 2015 to 2017, field experiments were conducted in three rainfall zones: low; intermediate and high; to quantify incidence and SED severity in Barbados. Additionally, fungal isolates were analysed by PCR using primers (SPM-4) designed from the <em>Smp450</em> gene cluster. A disease rating scale and digital assessment of disease severity were used to measure SED leaf symptoms, while cassava tuber size and mass were also measured, after production. Results showed high disease incidence (100%) in each zone with significant differences in disease severity between rainfall zones. No statistically significant differences were seen in tuber mass produced while tuber sizes were significantly different<em>. </em>Fungal isolates also displayed varying morphotypes between zones. A flat, smooth, mucoid orange color was common among isolates in low rainfall areas, while variation in fungal morphology with deeply fissured colonies was seen in isolates from the higher rainfall zone. Moreover, five PCR fragments ranging from 300- 150 bp were present in these latter isolates amplified with SPM4 PCR primers, but only four fragments (225-150 bp) were present in isolates from the low rainfall zone. These findings (varying patterns of disease severity, morphology and molecular characteristics) provide insights on<em> S. manihoticola</em> isolates and disease severity variation with rainfall and the management of SED in the cassava industry.</div>