INTERACTIONS WITH OTHER ORGANISMS: WEEDS, PESTS, DISEASES AND SYMBIONTS - Onions and other Vegetable Alliums - page 226

variations in virulence of different strains of the pathogen. In a study of tropical
onion cultivars, those with a high dry matter percentage and small bulbs were
most resistant to bacterial bulb rots (Swee-Suak Ko et al. , 2002).
Bactericides are used to prevent disease outbreaks and slow or prevent
epidemics, particularly of leaf blights. The most widely used bactericides are
copper-based, but zinc and some other metallic ions are bactericidal. Some
contain copper hydroxide, others chelated copper. Copper-based formulations
are preventative, while surface-acting bactericides and sprays need to be
initiated before infection enters the plant. There are reports of pathogens being
resistant to copper fungicides, and bactericides combining copper and maneb
(an ethylene bisdithioocarbamate (EBDC) fungicide) are currently the most
effective control strategy. In Colorado, applications every 5-10 days of this
combination as a high-volume spray to ensure complete leaf coverage is
recommended for the control of X. axonopodis pv. allii , starting 2 weeks before
bulb initiation. This results in eight or more sprays per crop. EBDC fungicides
are carcinogenic, and there is regulatory pressure to reduce and even cancel
their use in future (Gent and Schwartz, 2005).
Four sprays of acibenzolar-S-methyl - a chemical elicitor of plant systemic
acquired resistance (SAR) to pathogenic infections in general (Vallad and
Goodman, 2004) - were as effective in control of X. axonopodis pv. allii leaf
blight as nine to 12 sprays of copper plus maneb (Gent and Schwartz, 2005).
However, there is a 'metabolic cost' to the plant in activating its systematic
defences and, in the absence of disease, the yield of acibenzolar-S-methyl-
treated onions was about 24% lower than an untreated crop. Sprays of the
competitive bacteria Pantoea aglomerans and Pseudomoas fluorescens also
reduced infection, and biological control by bacterial antagonists may provide
another alternative to conventional bactericides in future (Gent and Schwartz,
2005).
FUNGAL LEAF DISEASES
There are a number of diseases caused by fungal pathogens that destroy the
leaves of edible alliums. These diseases were reviewed in detail by Maude
(1990a) and Schwartz and Mohan (2008). Additional information on some
diseases can be found in Sherf and Macnab (1986), Snowdon (1991) and
Maude (2006). Destruction of foliage leads to low yields of bulbs with shortened
storage potential. Leaf disease lesions on alliums harvested as leafy crops - for
example, salad onions - can lead to their rejection and a complete loss of crop.
Therefore, it is important to prevent these diseases. Leaf diseases divide roughly
into two categories, those prevalent in temperate regions and those of warmer
subtropical and tropical areas (Maude, 2006). The temperature ranges and
optima for pathogenesis and disease development reflect this division. Table 5.5
lists and summarizes some of the basic information regarding these diseases. It