Epidemiology of cucumber leaf spot diseases

Epidemics of leaf spot fungi were studied in 22 cucumber fields in North Carolina during summer--fall crops in 1997--1999. Epidemics were caused by Colletotrichum orbiculare, Didymella bryoniae, Corynespora cassiicola, Cercospora citrullina, Pseudoperonospora cubensis, Alternaria cucumerina, Ulocladium cucurbitae, Cladosporium cucumerinum, Helminthosporium sp., Stemphylium cucubitacearum, and Curvularia sp. This is the first report of Helminthosporium sp. as a leaf spot pathogen of cucumber. C. orbiculare and D. bryoniae were the most common pathogens found. The diversity of organisms was higher at the end of the epidemics than at their beginning.;Final disease incidence ranged from 58.93 to 99.42% and final disease severity from 16.27 to 69.39% from 1997 to 1999. The logistic model fitted 60% of the disease incidence data sets and 30% of the severity data sets in 1997 and 55.6% of incidence data sets in 1998. The Gompertz model fitted 40% of the incidence data sets and 70% of the severity data sets in 1997, 44.4% of the incidence data sets and all the severity data sets in 1998, and all incidence and severity data sets in 1999.;The beta-binomial distribution fitted the data (p ≥ 0.05) in 62.3 and 76.7% of the cases in 1997 and 1998 respectively. The binomial distribution fitted the data in 35 and 25% of the cases. Overdispersion (aggregation) was detected in ∼60% of the data sets in 1997 and 70% in 1998.;A simple, linear relationship between incidence (I) and severity (S) was derived where S = (0.011--0.013 I). This relationship remained constant among locations and seasons when disease incidence was <30%.;In 1999, final disease severity of four cucumber cultivars (Speedway, Dasher II, Thunder, and Poinsett 76) ranged from 4% on Poinsett 76 to 70% on Dasher II. D. bryoniae was the cause of >80% of the total severity on Poinsett 76. C. orbiculare was the cause of >90% of the total disease severity on Dasher II and Speedway. Thunder was intermediate in susceptibility to C. orbiculare and D. bryoniae . Poinsett 76 was resistant to C. orbiculare, but intermediate to D. bryoniae. U. cucurbitae was more severe on Thunder and Poinsett 76.