Abstract
Root rot of avocado trees (Persea americana Mill) caused by Phytophthora cinnamomi Rands is the most destructive disease affecting avocado trees worldwide. It is often associated to clay type of soils where prolonged saturation periods may often occur. The objectives of this study were to identify the species of Phytophthora associated to the root rot of avocado trees in Chile, and to study the effect of soil saturation periods on disease development. All isolates of Phytophthora obtained were identified as P. cinnamomi based on colony morphology and mycelium, sporangia and oospore characteristics. P. cinnamomi types A1 and A2 were identified. Regardless of the saturation period, inoculated avocado trees cv. Mexicola developed wilting, leaf chlorosis, leaf necrosis and defoliation. Inoculated plants had significant (p < 0.05) fewer leaves and lower leaf area per plant relative to non-inoculated plants. Furthermore, growth and trunk diameter were also significantly (p < 0.05) reduced on inoculated plants. Aerial symptoms were always associated to root rot and diseased plants eventually died. Saturation periods (soil matric potential = 0) every two weeks for 12, 24, 48 or 96 h resulted in 12.1 to 62.5% root rot on avocado cv. Mexicola. A linear regression best fit the relationship between the length of the saturation period and root rot incidence with Y= 24.5 + 0.89X (R2 = 0.86, p = 0.05) and Y = 2.20 + 0.16X (R2 = 0.66, p = 0.19) for inoculated and non-inoculated avocado trees, respectively. Based on these results, soil saturation periods of 24 h were enough to induce a 50% root rot on avocado cv. Mexicola. Despite the limitations to extrapolate these results to commercial conditions, repeated soil saturation periods higher than 24 h should be considered highly inductive to avocado root rot caused by P. cinnamomi under field conditions. La tristeza del palto (=aguacate) (Persea americana Mill), causada por Phytophthora cinnamomi Rands ocurre muy a menudo en huertos ubicados en suelos arcillosos y saturados por periodos prolongados. Este trabajo tuvo el proposito de caracterizar e identificar la especie de Phytophthora asociada a la tristeza del palto en Chile y de estudiar el efecto de la saturacion (potencial matrico = 0) del suelo sobre el desarrollo de esta enfermedad. Los aislamientos obtenidos correspondieron a P. cinnamomi, identificandose las razas A1 y A2. Paltos inoculados del cultivar Mexicola, independientemente del manejo del agua del suelo, desarrollaron marchitez, clorosis, necrosis foliar y tuvieron un significativo (p < 0,05) menor crecimiento, inferior numero hojas y una reducida superficie foliar respecto de los testigos sin inocular. Los sintomas aereos estuvieron siempre asociados con la presencia de pudricion parcial o total de las raices, lo que eventualmente provoco muerte de plantas. Los ciclos de inundacion realizados quincenalmente por 12, 24, 48 o 96 h tuvieron un efecto estadisticamente significativo (p <0,001) en el desarrollo de esta enfermedad en paltos del cultivar Mexicola inoculados con P. cinnamomi, los que desarrollaron entre 12,1 y 62,5% de pudricion radical, con relacion a plantas igualmente inundadas sin inocular. La relacion entre el tiempo de inundacion y la incidencia de pudricion radical obtenida por regresion lineal fue igual a Y = 24,5 + 0,89X (R2 = 0,86; p = 0,05) e Y = 2,20 + 0,16X (R2 = 0,66; p = 0,19) en paltos inoculados y sin inocular, respectivamente. De acuerdo con estos resultados repetidos periodos de inundacion por 24 h serian suficientes para producir un 50% de pudricion radical. En consecuencia, y a pesar de las limitaciones que pudieran existir para libremente extrapolar estos resultados a las condiciones de campo, es posible concluir que las inundaciones reiteradas y frecuentes del suelo, por periodos iguales o superiores a 24 h, son conductivas a un severo desarrollo de tristeza del palto, causada por P. cinnamomi.
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