620 Invasive Stink Bugs and Related Species (Pentatomoidea)
13.4.3 Legumes
Nezara viridula has been examined as a potential vector of bacterial blight of beans (Phaseolus vulgaris
L.) and cowpea [Vigna unguiculata (L.) Walp.], and various bacteria capable of causing necrotic water-
soaked spots on leaves of soybean [Glycine max (L.) Merrill] (Kaiser and Vakili 1978, Ragsdale et al.
1979). When only the stylets were allowed to touch artificial medium during feeding, field-collected
N. viridula adults in Louisiana transferred 31 types of bacteria representing 13 genera. Ragsdale et al.
(1979) obtained many of these bacteria from dissected insect bodies, but only isolates of Cornyebacterium
and Pseudomonas caused leaf vein necrosis and spots when applied to artificially damaged vegetative
soybean plants (Ragsdale et al. 1979). They concluded that only a “casual relationship” existed between
the stink bugs and the transferred microflora, but that N. viridula did have potential for significant vector
activity. Subsequent studies of field-collected soybean seeds exposed to different stink bug population
densities in Louisiana showed levels of seedborne bacteria to rise with increasing stink bug population
levels. However, this effect was seen only in the upper portion of plants, which suffered higher levels
of feeding damage, and only in 1 of the 2 years of the study. A similar but non-significant trend was
observed in the second year. Bug species present were predominantly N. viridula plus Chinavia hilaris,
and Euschistus spp.; bacterial isolates were not identified (Russin et al. 1988). In Puerto Rico, several
isolates of Xanthomonas axonopodis (as Xanthomonas phaseoli), which causes bacterial blight of beans
and cowpea, were obtained by washing field-collected N. viridula in sterile distilled water. Of 10 insects
tested, 5 carried Xanthomonas pathogenic to either bean or cowpea. Stem cankers formed when stink
bugs were exposed to a bacterial suspension and allowed to feed on cowpea, but no transmission was
observed when bugs collected from blight-infested plants fed upon caged test plants. In contrast, leaf-
feeding beetles did transmit the xanthomonads in controlled feeding trials. Therefore, the chrysomelid
Cerotoma ruficornis Olivier was suggested as the primary vector rather than N. viridula (Kaiser and
Va k i l i 19 78).
13.5 Fungal Pathogens
13.5.1 General Overview of Fungal Pathogens
Associations between pentatomid feeding and fungal pathogen transmission have been widely reported
(Mitchell 2004). Generally, fungi comprise the majority of plant pathogens (Agrios 2005). In contrast to
bacteria that typically require a host wound or other entry point, certain fungi produce structures called
appressoria that physically penetrate plant tissues (Mendgen et al. 1996). Once inside the host, necrotrophic
fungi absorb nutrients, killing the infected plant cells. In addition to filamentous forms, some fungi are
dimorphic growing as budding yeasts depending on environmental conditions (Webster and Weber 2007).
Fungal cells, with hyphae that are 55–100 μm long and 7–10 μm wide (Marasas 1971), are much larger than
bacterial cells (1.0–2.5 μm long by 0.8–1.0 μm wide), but they are still small enough to be internalized by
pentatomids. Here, we focus on several fungal diseases that are linked to vector-feeding activities.
13.5.2 Coffee
In coffee-growing countries of east Africa—including Uganda, Kenya, and Burundi—the pentatomid
species in the genus Antestiopsis are associated with numerous forms of damage to coffee plants (Coffea
arabica L.). According to Kirkpatrick (1937) and Le Pelley (1942), all Antestiopsis spp. are oligophagous—
feeding only on hosts in the Rubiaceae, although both studies were not comprehensive. It also appears
that C. arabica is a highly preferred host plant over numerous wild hosts as well as Coffea canephora
Pierre ex A. Froehner (= C. robusta) (Kirkpatrick 1937, Le Pelley 1942). Feeding by A. thunbergii
(Gmelin) (as Antestia lineaticollis Stål) on growing coffee shoots causes “die back” and reduced growth,
and feeding on green and red coffee cherries results in abscission of fruit (Le Pelley 1942). The role that
microbial pathogens play in shoot die-back or fruit abscission remains unclear.
Nonetheless, feeding by Antestiopsis thunbergii likely is associated with potato taste defect (PTD)
(Jackels et al. 2014). PTD disease is named for the moldy potato-like odor associated with diseased