Citrus - Crop Post-Harvest: Science and Technology - page 67

but is usually most severe on early maturing sweet orange
cultivars (Graham & Timmer 2009). In New Zealand it is a
serious problem on lemons and grapefruit (NZGI 1997). It
causes infection from the damping off of seedlings in nurs-
ery beds to decay of fibrous roots, crown rot, foot rot, gum-
mosis and the brown rot of fruits in groves and as
post-harvest decay during storage (Naqvi 2004).

(Futch & Timmer 2001; Barkley 2003). Fruit is most
susceptible to brown rot just after colour break. Symptoms
on fruit include a tan to olive brown leathery appearance
(Sommer et al . 2002). Under humid conditions, white
growth on the surface of the fruit may develop (Futch &
Timmer 2001). Brown rot from Phytophthora -infected fruit
can be distinguished from other brown rots by a character-
istic pungent spicy odor (Eckert & Eaks 1989). Infected
fruit usually drop but some may be picked unnoticed and
decay manifested post-harvest (Futch & Timmer 2001).

Morphology
The sporangia of P. citrophthora are more elongated than
P. nicotianae (45-90 × 27-60 μm) whilst P. nicotianae pro-
duce pear shaped to spherical sporangia (38-50 × 30-40 μm)
and abundant chlamydospores and oospores (22-29 μm)
(Timmer et al . 2000).

Control
Preventative measures
Skirting of trees reduces the opportunity for the soil-borne
inoculum to contaminate fruit in the canopy (Barkley
2003). The edge of the herbicide strip can also be main-
tained inside the dripline of the tree to minimize the expo-
sure of bare soil to direct impact by rain, which will limit
rain splash of soil onto the lower canopy. Precautions
should be taken during harvesting not to include fruit
affected by brown rot, in the field containers (Graham &
Timmer 2009). Other preventative measures include:

Disease cycle and epidemiology
Phytophthora species are able to survive unsuitable envi-
ronmental conditions over several years as dormant resting
spores (oospores or chlamydospores) (Timmer et al . 2000).
When environmental conditions become favourable, rest-
ing spores germinate. Sporangia release large numbers of
motile, biflagellate zoospores that can swim in short dis-
tances in water (Adaskaveg et al . 2008). Zoospores are
infective agents and form germ tubes on the fruit surfaces
that penetrate directly through the epidermis at any point of
the fruit surface. After 3-4 days at temperatures between
24°C and 28°C, brown spots become visible. At room tem-
perature after about seven days the entire fruit becomes
diseased (Eckert & Eaks 1989).
Susceptibility of fruit increases as the fruit mature and
the highest infection rate occurs just before harvest. Free
water is required for infection and the development of
brown rot depends on a long period of rain and fog.
Optimum temperature range for maximum fruit infection
and brown rot development was found to be 27-30°C with
wetness durations of 3h or more (Timmer et al . 2000).

• Proper irrigation management.
• Mowing to prevent growth of ground vegetation.
• Pruning to remove low hanging branches.
• Maintain adequate soil drainage to limit contamination
of fruit with soil-borne inoculum (Barkley 2003).

Chemical control
Aliette, Phostrol and ProPhyt are systemic fungicides that
protect against post-harvest infection and can provide con-
trol for 60-90 days. Copper fungicides can also be applied
and are primarily protective, but are also capable of killing
sporangia on the fruit surface thereby reducing the level of
inoculum (Graham & Timmer 2009). Copper sprays
applied particularly to the skirt and the under-tree areas are
also effective in preventing brown rot (Hardy 2001).
A potassium phosphite product (Canon ® ) was evaluated
for the control of brown rot on oranges and grapefruit and
exhibited effective disease control (Oren & Yogev 2002).

Transmission
During prolonged periods of rain and fog, the fungus spor-
ulates on the soil and infected fruit and the motile spores
(zoospores) are dispersed by rain splash and air currents to
low hanging fruit on the tree. Infection is therefore gener-
ally limited to the lower third of the tree canopy (Sommer
et al . 2002).

Black spot
Citrus black spot (CBS) is caused by Guignardia citri-
carpa Kiely and is a fruit disease that affects the rind, but
does not cause internal decay (Timmer et al . 2000). Black
spot attacks leaves, branches and particularly fruit of sweet
oranges, lemons, grapefruits, some tangerines and several
hybrids (Ayres 2001). Lemons are particularly susceptible
(FFTC 2003) and heavy losses may also occur on Valencia

Symptomology
The disease is not very common, but all cultivars can be
affected with early varieties showing greater susceptibility,
especially during long, rainy periods. Highest infection
occurs on fruit that mature at the time rainfall occurs