Sporulation of Phytophthora palmivora (Butl.) Butl. in relation to epidemiology and chemical control of cacao black pod disease / Arnaldo Gomes Medeiros.

Tese. (P.h.D.) - University of California, 1976

The cacao black pod disease, caused by the fungus Phytophthora palmivora, accounts for a 30 percent annual loss world wide in the cacao crop. This research was aimed to develop scientific bases for the control of black pod. The progress of black pod and the temperature, humidity, and rainfall were recorded daily at two sites in the same plantation (Bahia, Brazil). The epidemic followed a compound interest pattern (r =0.03 to 0,06/unit/day) and was correlated with vapor pressure deficit and with the rainfall four days proceding the occurrence of black pod. The frequency of infection was one lesion/pod/equatorial zone and the latent period was correlated to temperature. When the sites of natural inoculum were determined by collecting samples and inoculating cacao pods and by isolation in a selective medium the fungus was detected mainly in soil, mummified pods and in husk piles. The infection potential in soil fluctuated seasonally. The highest amount of propagules (75 x 10³/g) was found 20 cm from the cacao tree. Propagules were also detected in rain water, splashed from soil and running down the cacao trunk, but not in the air. Exposure to light (cool white Westinghouse fluorescent lamp 200 ftc) at 15ºC provided the maximum sporulation and detachment of zoosporangia. Single zoospore isolates responded similarly to hyphal tip isolates. When rinsed colonies were suspended in buffer solutions, sporulation was suppressed to some extent. When potassium phosphate (5 x 10-³M) was dripped on colonies, the optimum sporulation occurred at pH 6.40 to 6.80, the same as in the black pod lesion. Sporulation was suppressed by an excess of glucose (10 elevado a 4 ug/ml), stimulated by manganese (1 ug/ml) and copper (0.1 ug/ml) ions, affected by cacao cultivar and age of pod and correlated with total reducing sugar in the husk. In a sealed plate sporulation increased when the concentration of CO2 reached 0.2% and when agar was dehydrated; in a black pod lesion sporulation occurred in an acerculus-like structure and dehydration cooled the pod surface. Seventy six chemicals were tested for antisporulant effect in culture and on infected cacao pods, Aspartic acid, canavanine, ethionine, salicylic and cinnamic acids, cyclohexamide, malachite green and copper sulfate were active inhibitors in both assays. Urea, sucrose, adenine, cytosine, and hydroquinone were active only on the pods. In the field assays urea (10%) repressed sporulation 100% in sprayed infected husk piles, while a copper fungicide suppressed 10% sporulation in infected pods; the activity was not improved by the addition of malachite green or cyclohexamide. Spraying the cacao tree with a copper fungicide at high volume (160 liters/ha/application or 4.0 g/CU++/tree/application) was effective even when pods were not aprayed. One fungicidal spray between the two cacao seasons delayed the progress of the black pod epidemic for 21 days, but when the disease level exceeded 1%, sanitation (periodical removal of infected pods) was the only feasible measure. When mineral spray oil was used with copper, chemical control by low volume sprays (20 liter/ha/application) was an effective as by high volume sprays when the infection rate was 0.06/unit/day. Copper/ha/application was reduced 47% and less labor was required for application. It will be possible to compare results of field experiments on black pod control at different locations in the world by expressing the disease severity as the infection rate of the epidemic.