Despite numerous bactericide applications, significant yield loss is not uncommon depending on the disease pressure and cultivar susceptibility. As a Master's student in the department of Plant of Plant Pathology and Environmental Microbiology at Penn State, much of my research was focused on refining bacterial spot management. Specifically, I examined the epidemiology of bacterial spot, the defoliation associated with the disease, and age related resistance in fruit.
Epidemiology is the study of disease over time or space. It can be used to model yearly epidemics and forecast infection periods. Also known as disease progress over time, it can be used as a tool to allow us to better target our management efforts. When we looked at the disease progress of bacterial spot, we found a lot of variability at the tree level. In other words, trees of the same cultivar, receiving the same chemical treatments had varying disease progress over time. Although this made disease progress difficult to model, we found that treatment and cultivar were significant factors influencing disease progress. It made sense to us that cultivar was a significant factor because it confirmed that some cultivars are generally more susceptible than others. When we looked at the treatments (the reports of these trials have been previously reported so I won't report them again here) we found that the standard oxytetracycline (i.e., Mycoshield or FireLine) was not significantly different than the untreated trees. This made no sense to us because, anecdotally, treated trees usually have less disease than untreated trees. We felt our results had likely been influenced by the severe defoliation that occurred in the experiment block at the extension center. Because only leaves that remain on the tree can be assessed for disease progress, the heavily infected leaves that likely dominated the untreated trees fell to the ground, artificially lowering disease progress to similar levels of the treated trees.
Severe premature defoliation is not an uncommon symptom of bacterial spot. Despite that, it had not been previously studied. Using two survival analysis methods, common techniques often used in clinical medicine to determine the time to death of patients in clinical trials, we were able to examine the factors influencing defoliation. We followed 3,052 leaves throughout the growing season and found that just under half of them (1,460 leaves to be exact) fell off the tree before fruit harvest. The factors we examined were leaf age, cultivar, treatment, and disease severity at initial disease onset. We found that older leaves abscised significantly faster than younger and middle-aged leaves. This made sense because older leaves have more time to accumulate higher levels of disease and more opportunity to fall from the tree. We also found that cultivar influenced defoliation and that leaves from the cultivars 'Beekman' and 'Snow King' abscised faster than the leaves of 'Easternglo' and 'Sweet Dream'. This was interesting because 'Sweet Dream' and 'Snow King' tend to be more susceptible to bacterial spot than the cultivars 'Easternglo' and 'Beekman'. This implies that defoliation is not a good indication of overall bacterial spot susceptibility. We then found that chemical treatment did not influence defoliation. However, we do know that injury caused by copper containing compounds will likely make defoliation worse so take care when applying those chemicals. Nevertheless, when we looked at disease severity at initial disease onset, we found that for every small increase in initial disease, the time it took the leaves to abscise was greatly reduced. This means that disease severity at initial disease onset is a critical target for focusing management strategies in order to reduce leaf abscission. You can target initial disease by reducing the amount of initial inoculum in the beginning of the season by pruning out bacterial spot cankers in the early spring as well as with a dormant copper application.
Age Related Resistance in Fruit
Finally, in an attempt to reduce the overall number of bactericide applications, we examined age-related resistance or the possibility that fruit become less susceptible to bacterial spot infection as they mature. We thought if we could control the developmental stage at which fruit were exposed to infection, we could find an approximate date when fruit no longer developed symptoms of bacterial spot after being exposed to infection. In fact, we found that there was an exponential decline in susceptibility as fruit matured throughout the season. Fruit exposed to infection at the beginning of the season were heavily diseased while those that were exposed after the end of June had almost no disease, even without the use of antibiotics. This could help reduce the overall number of antibiotic applications needed each year, reducing not only management costs, but the risk of antibiotic resistance, as well. However, there are a few limitations to this study. The cultivar used in this study was 'Beekman' and it is unknown if this theory would hold up for all other stone fruit cultivars. In addition to that, the date when fruit are no longer susceptible to bacterial spot infection would likely vary from year to year depending upon the weather and overall disease pressure. Nevertheless, this study indicates that the beginning of the season is when fruit are most susceptible to bacterial spot and is therefore the most critical time of the season to focus bacterial spot management efforts.
As with defoliation, reducing initial inoculum will also help protect young susceptible fruit.
The author would like to acknowledge Dr. Henry K. Ngugi for his participation in this research as well as her current thesis advisor, Dr. Maria del Mar Jimenez-Gasco.