Although relatively rare, some viru

Although relatively rare, some viruses and viroids can be transmitted
within pollen to ovaries during pollination and subsequently
to the progeny seed and seedlings (23). For those virus– or viroid–
host combinations, pollinating insects can thereby aid in plant-toplant
spread and infection of a healthy plant (20,65). Prophylactic
blossom removal before flowering in nursery plantings in areas
where pollen-transmitted viruses and viroids are common is a recommended
management practice to reduce pathogen dispersal by
pollen. Many countries have strict phytosanitary regulations to
ensure that pathogens are not introduced in pollen that is traded
internationally as a source of germplasm by breeders (92).
Accurate detection of pathogens is essential for disease management.
Detection and identification of systemic pathogens poses
several challenges: (i) false negatives due to low pathogen concentration
in plants; (ii) uneven distribution among tissues and organs
within a host; (iii) latent or subliminal infections; (iv) lack of pathogen
reference collections or data to represent the genetic diversity
needed to validate diagnostic tests; (v) lack of validated sampling
and testing protocols capable of detecting a broad range of pathogen
isolates; (vi) the length of time required to obtain some test
results; (vii) the need to adopt and employ newly emerging, rapidly
evolving diagnostic methods; and (viii) the lack of robust tests that
are easily and economically conducted for large sample sizes
(56,82).
Efforts to overcome these challenges have led to the development
of diverse detection methods, including biological assays on
indicator plants, electron microscopy, serological assays such as
enzyme-linked immunosorbent assay (ELISA) and tissue blots, and
molecular assays such as nucleic acid hybridization and polymerase
chain reaction (PCR) variants (56). The preferred method for
detection of a particular pathogen often changes over time as improved
methods are developed and validated, and frequently multiple
methods are used in order to satisfy phytosanitary requirements.
Continuing advancements in detection technologies are
probable in the form of macroarrays (101) employed as a single
test to detect the most common viruses in a host plant, and “Deep”
or “Next generation” sequencing as a diagnostic tool to detect the
presence of known or even new pathogens (44,52,95). Reviews of
current and promising new methods for detection of systemic pathogens
have recently been published (1,28,41,43,56,61,95) and will
not be reiterated here.
History and socio-economic consequences of some notable
epiphytotics caused by systemic pathogens of fruit crops. Following
are descriptions of three diseases of fruit crops (plum pox,
citrus tristeza, and huanglongbing) to illustrate the devastating
socio-economic consequences that systemic pathogens can inflict
on fruit industries. The costs include: (i) direct crop loss realized
by producers, and (ii) disease management strategies that may
include surveillance, quarantine, eradication and replanting, vector
control, production and maintenance of healthy planting stock, and
research. Numerous additional examples of epiphytotics caused by
systemic pathogens can be found in the literature (3,31,39,58,61,
91,102)