Flyspeck Epidemics I: IVIeasuring 

 Ascospore Maturation of the Causal 

 Fungus 



Daniel R. Cooley, Susan M. Lerner, and Arthur F. Tuttle 



Department of Plant, Soil, & Insect Sciences, University of Massachusetts 



If flyspeck of apple were like apple scab and had 

 both a primary stage and a secondary phase, it might 

 be useful to understand how the primary stage works, 

 so that management tactics could focus on it, the same 

 way scab management is focused on primary scab. 

 Theoretically, if the epidemic could be stopped early, 

 then summer fungicides might be greatly reduced. 

 Interestingly, the fungus that causes flyspeck, 

 Schizothyrium pomi and its asexual form, Zygophiala 

 jamaicensis, are ascomycetes that are similar to the 

 apple scab pathogen Venturia inaeqiialis. That is, S. 

 pomi produces ascospores and conidia; however, not 

 much more is known about how the fungus overwinters 

 and then ends up producing the damaging black specks 

 on apple fruit. Of course, it grows on the wa.xy cuticle 

 of fruit fomiing colonies of circular, black, specks 

 called thyriothecia. These thyriothecia group in 

 colonies of several to 50 or more, and it is these 

 structures that eventually produce ascospores. There 

 are a few key questions to ask. Do thyi'iothecia produce 

 ascospores throughout the year, or just during a single 

 period? Secondly, if there is just one period of 

 ascospore production, when is it? Finally, where do 

 the ascospores land and infect? Nearly 8 years ago, 

 we suggested that ascospores matured only once a year, 

 during the spring and early summer and that they might 

 be the spores that start flyspeck epidemics each year 

 (7). Now we have more definitive information on how 

 ascospores of the flyspeck fungus function. 



When evaluating the risk of apple scab infections 

 in the spring, it is useful to look at the fungal structures 

 that contain ascospores, to see how mature the spores 

 are. The more mature spores get, the higher the risk 

 goes. Such an approach might prove useful in both the 

 study and, eventually, management of FS. As part of 

 this study, we modified a technique for the preparation 

 and interpretation of thyriothecial squash mounts of 5. 



pomi. 



Flyspeck can live on the waxy cuticles of many 

 kinds of plants. We identified S. pomi on 26 woody 

 plant species that commonly grow m orchard borders 

 m Massachusetts including trees, shrubs, and vines. 

 For these studies, we have focused on a common 

 blackberry (Rubus allegheniensis Porter), which 

 supports easily identifiable infections with abundant 

 thyriothecia. Blackberry is an excellent indicator of 

 the presence oiS. pomi near orchards and is one of the 

 most common hosts of the fungus in Massachusetts. 



Thjriothecia can be picked from blackberry, using 

 a dissecting microscope, and then squashed using a 

 method similar to the one used for apple scab. They 

 are examined under a high-powered microscope, and 

 rated according to the following maturity classes: 

 undeveloped, no asci present (0); immature asci present 

 without ascospores ( 1 ); mature asci present containing 

 ascospores (2); majority of asci ruptured or empty with 

 or without released ascospores (3). 



The blackberry canes can also be cut and brought 

 into the laboratory, where they can be put into 

 controlled environments to see how the thyriothecia 

 develop. So, over several years, canes have been 

 incubated in different humidity levels and at different 

 temperatures. 



Both temperature and humidity significantly affect 

 the maturation of S. pomi ascospores. Thyriothecia do 

 not produce ascospores when the air is not extremely 

 humid, 99% relative humidity or more. It also needs 

 to be relatively warm for ascospores to mature. At 

 70^, if the air stays nearly saturated with humidity, S. 

 pomi ascospores will mature in about 48 hrs. At 57°F, 

 it takes 5 days, and at SO^F it takes 6 to 9 days. Below 

 48°, thyriothecia never really develop m the lab, even 

 after 18 days of incubation (Table 1). 



We followed maturation of thyriothecia on canes 



Fruit Notes, Volume 69, Summer, 2004 



