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L. W. Timmer

L. W. “Pete” Timmer was born in West Olive, MI. He received a B.S. degree from Michigan State University and a Ph.D. degree from the University of California, Riverside. In 1970, he accepted a position as assistant professor at the Texas A&I Citrus Center in Weslaco and was later promoted to associate professor. In 1978, he accepted a position at the Citrus Research and Education Center (CREC), University of Florida, Lake Alfred. He was promoted to full professor in 1982.

Pete has developed a comprehensive research program on the etiology, epidemiology, and management of diseases caused by all of the major groups of citrus pathogens. He described citrus ringspot virus, demonstrated its relationship to psorosis bark scaling, and collaborated on work on mechanical transmission and purification studies, which led to the proposal of a new virus group. With Phytophthora diseases, he developed methods for evaluation of fungicides, developed quantitative assays for Phytophthora populations, and demonstrated the relationship of populations to root rot and yield loss. In Florida, one of the greatest challenges has been dealing with citrus blight, a serious decline disease of unknown etiology. With his colleagues at CREC, he developed diagnostic techniques to differentiate blight from other decline diseases and demonstrated root-graft transmission.

In 1983, a new disease, postbloom fruit drop (PFD), appeared in Florida citrus groves. Initially, PFD was confined to Tahiti lime plantings in south Florida. In 1988, PFD became widespread on sweet oranges and Dr. Timmer, with his students and associates, initiated an extensive research program to determine (i) the nature of causal strains of Colletotrichum, (ii) the disease cycle, (iii) the effects of environmental factors on the disease, and (iv) how to develop predictive models for effective timing of fungicide applications. Dr. Timmer’s prolific research program culminated in 11 publications from 1992 to 1996 in Phytopathology and Plant Disease, and 8 in citrus industry publications.

Timmer and J. P. Agostini confirmed PFD is caused by a specialized strain of Colletotrichum, designated slow-growing orange (SGO). They differentiated the ubiquitous C. gloeosporioides strain, fast-growing gray (FGG), from the SGO strain by morphological characters, including conidial shape, setae presence, and appressorial shape. Interestingly, they discovered the SGO strain was almost identical to Gloeosporium limetticolum, which causes anthracnose on Key lime. Isolates of the fungus from Key lime, KLA strains, produced blossom blight and persistent buttons (fruit pedicels remaining after fruitlet drop) as did SGO strains, but SGO isolates did not produce typical Key lime anthracnose. Timmer and Agostini theorized that PFD developed when the fungus moved from Key limes into sweet orange groves and attacked blossoms. Both the SGO and KLA strains have recently been identified as Colletotrichum acutatum.

Through laboratory and field studies, Timmer and associates worked out the disease cycle of PFD. The SGO strain produces abundant conidia in acervuli on diseased petals. Conidia of the SGO strain germinate to form appressoria on the surface of living leaves, whereas the FGG strains produce appressoria that penetrate leaves and invade tissue after it becomes senescent. Timmer and students observed that the SGO strain produces appressoria that persist on the leaves but do not colonize senescent or dead tissue. Instead, they germinate in the presence of moisture and blossom extracts and produce conidia borne singly on hyphae. These conidia serve as the inoculum for infection of the first few flowers, which reinitiates the disease cycle in the spring.

Next, Timmer and colleagues examined the interaction of environment and pathosystem. They confirmed that free water is required for infection, but more importantly, rain splash and wind-blown rain are needed for dispersal from conidia. Disease incidence is more strongly related to rainfall prior to infection than to leaf wetness. Consolidating knowledge of disease cycle and environmental effects, Timmer and Zitko conducted a series of studies of fungicide efficacy and timing. Because adequate control with benomyl is highly dependent on timely applications, they developed an equation for prediction of disease incidence 3 to 4 days in advance. The model has been widely adopted by the citrus industry to determine whether and when to spray.

Dr. Timmer’s presentations and articles have emphasized the need for accurate information on which to base spray decisions: knowledge of stage of bloom, PFD level, and accurate local rainfall records. Losses due to PFD tend to be overestimated because of the large number of buttons produced. Fungal infection induces the formation of a button regardless of whether the flower would actually set a fruit. Timmer and Zitko found that trees tend to compensate for fruit lost to PFD by dropping less fruit during normal physiological drop. The number of buttons remaining from the previous season are a good early season predictor of disease severity in the current season. Furthermore, there is a strong correlation between number of buttons in the current year and number of fruit set. Timmer’s model-based decisions on fungicide application result in large, economically important increases (25 to 500%) in fruit production.

PFD has become an increasingly prevalent and locally disastrous problem in the Caribbean and South America. Dr. Timmer is called on regularly to provide educational programs, particularly in Spanishspeaking countries, and to consult with individual citrus growers. In 1994, he lectured to over 1,200 people at a symposium specifically held on PFD in Sao Paulo, Brazil.

Dr. Timmer is an avid birder. He participates in many bird counts and is active in conservation organizations. He has seen and identified over 2,000 species of birds worldwide.