Integrated Management of Viral Diseases in Field-Grown Tomatoes in Southern Italy

Tomato spotted wilt virus (TSWV) is a Tospovirus affecting field tomato crops worldwide. The virus is transmitted by some species of thrips. The most damaging vectors in Italy are Frankliniella occidentalis and Thrips tabaci. Only the first instar larvae can acquire the virus from an infected plant. After acquisition, the virus replicates in the vector, and the viruliferous thrips is capable of transmission for the duration of its life. Apart from TSWV, other serious threats for tomato production in Italy are the aphid transmitted Cucumber mosaic virus (CMV) and Potato virus Y (PVY), both with necrotic and non necrotic variants. Epidemics caused by TSWV, PVY and CMV occur regularly in southern Italy, where hundreds of millions of tomato plants are grown commercially every year. Insecticides applied on a calendar basis for insect control are not very effective in preventing these vector-borne viral diseases because the viruses can be transmitted to the plant before insects (thrips and aphids) are killed by insecticides. Field experiments were conducted during summer of 2005 and 2006 in the main tomato growing area of the Basilicata region, Lavello (PZ), to determine the separate and integrated effects of UV-reflective mulch (UVRM), Acibenzolar-S-methyl (Actigard) and insecticides for the management of vectors and diseases on tomato. TSW disease pressure was great during 2005 summer but very low in 2006. Disease incidence was measured as percentages of plants infected with each of the three viruses. Aphid populations were evaluated by counting the number of aphids on sampled tomato shoots on one third of the plants on each plot. Tomato yield was evaluated from one third of the plants on each plot. Statistical analysis (analysis of variance) was performed using the SYSTAT 9 Software. The UVRM alone was effective in reducing disease incidence both for aphids and thrips transmitted viruses. A synergistic effect of Actigard with UVRM was observed against the three viruses. The use of the insecticides resulted in a further reduction in the incidence of the three viral diseases on UVRM, and reduced incidence of diseases caused by CMV and PVY in the black mulch and bare soil plots. The combination of UV-reflective mulch, acibenzolar-S-methyl, and insecticides was the most effective in reducing tomato spotted wilt incidence in tomato. The UV-reflective mulch was very effective in delaying and reducing colonization of aphids and subsequent virus incidence. There was no significant difference in the tomato yield between UVRM and black mulch plots. In contrast, yield was strongly reduced in the bare soil plots. a [email protected] Proc. II Intl. Symposium on Tomato Diseases Eds.: H. Saygili et al. Acta Hort. 808, ISHS 2009 388 INTRODUCTION Tomato spotted wilt virus (TSWV) is a Tospovirus that adversely affects field tomato crops worldwide. The virus is transmitted exclusively by some species of thrips. The most damaging vectors in Italy are Frankliniella occidentalis and Thrips tabaci. Only the first instar larvae can acquire the virus from an infected plant. After acquisition, the virus replicates in the vector and the viruliferous thrips is capable of transmission for the duration of its life. Apart from TSWV, other serious threats for tomato production in Italy are the aphid transmitted Cucumber mosaic virus (CMV) and Potato virus Y (PVY), both with necrotic and non-necrotic variants. Epidemics caused by TSWV, PVY and CMV occur regularly in southern Italy, where hundreds of millions of tomato plants are grown commercially every year. Insecticides applied on a calendar basis for insect control are not very effective in preventing these vector-borne viral diseases because the viruses can be transmitted to the plant before insecticides kill the insect vectors (thrips and aphids). Therefore, field experiments were conducted during summer of 2005 and 2006 in the main tomato growing area of the Basilicata region, Lavello (PZ), to determine the separate and integrated effects of UV-reflective mulch (UVRM), Acibenzolar-S-methyl (Actigard) and insecticides for the management of vectors and diseases on tomato. MATERIALS AND METHODS A randomized complete block experiment with four replications was conducted in the summer of 2005. The same experiment was repeated in 2006 again with four replicates. The tomato crop was produced using typical commercial practices. A splitsplit-plot treatment arrangement was used to determine the separate and combined effects of each tactic on the efficacy to reduce TSWV, CMV and PVY incidence. Tomato cultivar was ‘ULISSE’. Six-week-old transplants were spaced every 33 cm in raised beds covered with UV reflective mulch (UVRM, treatment A), black plastic mulch (BPM, treatment B) or no mulch (NM, treatment C). Plot size for each split-split plot was 3 twin-rows 5 m long by 5 m. The whole plots were separated by a 1.5 m buffer zone. Insecticide and Acibenzolar-S-methyl treatments are reported in Table 1. TSW disease pressure was great during 2005 summer and very low in 2006. For this reason, only results of the 1st year experiment are reported. Percent disease incidence was determined by visual inspection of each plant in the two middle rows of each plot. A week after disease assessment date, TSW visual symptoms were verified from 5 symptomatic plants/plot using ELISA. The same was done to assess CMV and PVY disease incidence. Aphid populations were evaluated by counting the number of aphids on sampled tomato shoots on one third of the plants in the middle of each plot. Tomato yield was evaluated from one third of the plants on each plot. Statistical analysis (analysis of variance) was performed using the SYSTAT 9 Software. RESULTS AND DISCUSSION Evaluation of Aphid Infestations Three classes of aphid populations were identified: 1) number of aphids = 0 (indicating no infestation) (Fig. 1a); 2) number of aphids < 10 (indicating a delay in aphid colonisation) (Fig. 1b); number of aphids > 10 (aphid colonisation) (Fig. 1c). UVRM determined a strong control of aphid infestations. We suppose that this was due to aphids being disoriented by the UVRM, which prevented the primary infestation of tomato plants and the consequent colonisation of the same. On black mulch, we observed an early infestation of aphids compared with the no mulch treatment, maybe due to the absence of weeds, which could have had a role in the primary diffusion of aphids. On bare ground (treatment C), by contrast, the presence of weeds at the beginning reduced the number of aphids on tomato vegetation (aphids being distributed between tomato and weeds), and subsequently the same weeds represented a reservoir of insects for the secondary infestations. In the UVRM treatment, moreover, no significant differences among the different insecticide and Actigard treatments were detected, meaning that the UVRM was