Effect of Bloom Sprays on Alternaria Rot in California Mandarins and Fungicide Efficacy Against <i>Colletotrichum</i>

This study aimed to provide new insights into the management of Alternaria rot and Colletotrichum dieback in mandarins. Field trials were conducted to evaluate the impact of bloom sprays on latent infections, disease progression, and the influence of bloom inoculations on disease development. Detached citrus leaves were used to assess the efficacy of commercial fungicides in reducing infections by <i>Colletotrichum</i> spp. <i>C. karstii</i> was more sensitive than <i>C. gloeosporioides</i>, as evidenced by the overall probabilities of infection. Preventative treatments containing quinone outside inhibitor fungicides, such as Pristine, Quadris Top, and Luna Sensation, consistently reduced infections. Curative treatments with Pristine and Luna Sensation also demonstrated efficacy. Bloom inoculations with <i>Colletotrichum</i> species did not induce visible symptoms on flowers and twigs, nor did they lead to dieback until harvest, suggesting that bloom infections may not play a significant role in the disease cycle. For <i>Alternaria</i>, the frequency of latent infections in flowers increased significantly during the bloom period. Shoots inoculated with <i>Alternaria</i> species during bloom reduced the number of fruits per shoot at harvest. Fungicide treatments during bloom exhibited a dual effect, reducing <i>Alternaria</i> infections in fruitlets and influencing disease progression, ultimately reducing preharvest fruit drop. This study provides valuable insights into the role of flower infections in the development of Alternaria rot and Colletotrichum dieback in California mandarins. Furthermore, it offers insights into the efficacy of commercial fungicides in these pathosystems.