Pecan scab, caused by the fungus Cladosporium caryigenum (Ell. et Lant) Gottwald, produces more damage to pecan than all other diseases and insects combined. Early events during infection are critical to disease establishment and to expression of host resistance, but have not been examined previously. Objectives of this research were to determine if there is regulation of appressorial formation and if it is related to resistance. Pre-infectional host-pathogen interactions were studied in vivo (on leaves) and in vitro (on callus, dialysis membrane, and agar) with light and electron microscopy. Leaves, callus tissue, dialysis membranes, and agar were inoculated with scab conidia and were incubated under conditions optimum for germination. Conidia germinate and produce a germ tube on agar and dialysis membrane, but appressoria are not formed. Appressoria form on pecan callus, but germ tubes are long. Long germ tubes are often associated with resistant disease reactions. In vivo, appressoria form readily, but germ tube length varies depending on the location of the spore on the leaf surface. Preliminary evidence indicates that surface topography affects induction of appressorium formation in the scab fungus.
K.M.T. Cason and I.E. Yates
I.E. Yates, K.M.T. Cason, and Darrell Sparks
Leaves and callus of pecan [Carya illinoinensis (Wangenh.) K. Koch], and glass, dialysis membrane, and agar were examined for capacity to support two of the earliest infection stages—conidium (spore) germination and appressorium formation—of Cladosporium caryigenum (Ellis & Langl.) Gottwald, the fungus causing pecan scab. Light and temperature effects on formation of germ tubes and appressoria were examined for conidia suspended in distilled-deionized water. Conidia formed germ tubes on all substrates and in distilled-deionized water; hence, conidia possessed endogenous materials required for germination and are independent of specific topographic or chemical stimuli. All substrates, except 2% water agar and water, sustained appressoria development, thus implicating regulation by surface hardness. More appressoria formed on leaf discs than on other substrates. Additionally, conidia formed appressoria with short germ tubes when near a leaf structural feature, such as stomatal guard cells. Thus, the pecan scab fungal isolate used in these experiments appeared to lack substrate specificity for forming germ tubes, but not appressoria, during the prepenetration stages of development. Conidium germination was maximized at about 25 °C and germination did not respond to light.