Septoria leaf spot in organic tomatoes under diverse irrigation systems and water management strategies

The objective of this study was to analyze the temporal progress of septoria leaf spot of tomato, caused by Septoria lycopersici, in different irrigation systems. The experiment was carried out in an organic farming system, in a randomized block design with three replicates. Six irrigation systems were evaluated: GO(1L): one drip-line per row of plants; GO(2L): two drip-lines; SU: furrow; MIsub: microsprinkler below plant canopy; MIalta: microsprinkler above plant canopy; AS: overhead sprinkler irrigation. Water management strategies varied with soil moisture tensions. Irrigation by drip systems was carried out when soil water tensions reached 15-30 kPa (high moisture), whereas for SU the trigger soil tension was 30-60 kPa (moderate moisture). All other systems were conducted at both soil water tension ranges. One additional treatment was conducted with overhead sprinkler, high soil moisture and straw mulch (AS(palha)). The disease manifested itself by natural infection, and quantification was carried out weekly from the onset of symptoms. Disease severity was estimated at the halfway point (Y-50) and at the end (Y-max) of the time-course of the epidemics, and the area under the disease progress curve was also calculated (AUDPC). Disease progress analysis was done after fitting severity data to the Gompertz model. Highest Y-max (30-35%) values were found in the AS systems, while the lowest severities were associated with MI (6-10%), GO and SU (4%), indicating that the smaller the water drop size, the lower the disease severity. To a lesser extent, higher frequencies of irrigation in the AS and MI systems also favored higher disease levels. The use of straw mulch (AS(palha)) significantly reduced Y-50 and AUDPC, when compared to AS.