Downy mildew and angular leaf spot are both damaging foliar diseases of cucurbits, especially in cucumbers. These diseases can look very similar in the field, but require different management tactics for effective control. Because of this, it is critical to correctly identify which disease you are dealing with. This blog discusses the the two diseases and how to differentiate them.
Downy Mildew:
Downy mildew of cucurbits (Pseudoperonospora cubensis) was not considered a major threat in North America until 2004 when the disease reemerged and caused significant damage to cucumber varieties previously thought to be resistant to the disease. Downy mildew of cucurbits was first detected in Nova Scotia in 2021, and has appeared every year since. The disease does not overwinter in Nova Scotia or the North Eastern United States, and is spread from the Southern United States each summer by storms. Cucurbit downy mildew can be tracked at this site (https://cdm.ipmpipe.org/), as of July 19, 2024 the disease has only progressed as far North as Pennsylvania and New York, and has not been detected in Nova Scotia yet this season. The disease can significantly damage cucurbit crops if there are no management techniques in place.
The signs of downy mildew include lesions on the leaves (Figure 1).
Angular, vein-delimited lesions that may initially appear slightly chlorotic
are typical disease symptoms on cucumbers, and pathogen sporulation is easily
visible on the lower leaf surface. Under humid and moist conditions, early
infection lesions on cucumbers may appear water-soaked. The bottom leaf surface
may appear "dirty" due to the sporangial masses' grey to purplish
coloration when they are present in large numbers. These dark sporangial masses
on the underside of the leaf are unique to downy mildew of cucurbits, and the
underside of the leaf should always be examined to distinguish between downy
mildew of cucurbits and angular leaf spot. Lesions become more chlorotic and
necrotic as the disease worsens, leaves begin to die and the plant is unable to
support fruit production. It is possible that both the quality (misshaped
fruit) and quantity of marketable yield will decrease.
Figure 1. Downy mildew on cucumber leaves. Note the dark, spongy masses on the back of the leaf.
The pathogen was found to be resistant to the strobilurin
fungicides (FRAC class 11) when it reappeared in 2004. While reports of
decreased performance of particular fungicides demonstrates the need for
resistance management and the development of new and unique active ingredients,
newer downy mildew fungicides have been useful in containing the disease. The
use of resistant cultivars and fungicide treatments should be paired with
cultural practices to decrease downy mildew incidence. Increased row and plant
spacing, drip watering and elevated plant beds with plastic mulch can all
assist in limiting the pathogen by reducing leaf wetness, boosting airflow, and
lowering the relative humidity.
Angular Leaf Spot:
The most common bacterial disease of cucurbits is angular
leaf spot, caused by the bacterium Pseudomonas syringae pv. lachrymans.
Disease severity is greatest in cucumber cultivated in warm, humid and
semi-humid climates, but it affects a wide variety of cucurbit
crops worldwide. Initially infecting cotyledons and leaves, the disease later
spreads to fruits and contaminates seed.
Small, water soaked lesions first emerge on leaves as the disease
spreads (Figure 2). The lesions typically grow until they are surrounded by substantial
secondary veins in the leaves, which give them an angular appearance. A clear
to milky exudate that gathers in a tear on the lower surface of the lesion
occurs in conjunction with water-soaking at high relative humidity levels. On
or near the lesion, the exudate dries to produce a thin, white crust. The
blemishes on the leaves grow dry, tan-brown, and may even drop out. Lesions on
cultivars that are sensitive frequently have yellow edges. In areas between
lesions, heavily diseased leaves may turn yellow. Later, they may appear frayed
or ragged. Petioles, stems and fruits may also experience lesions, showing a
white, crusty discharge. This white, milky exudate is the defining
characteristic of angular leaf spot and can be used to distinguish the disease
from the very similar-looking downy mildew of cucurbits.
Figure 2. Angular leaf spot on cucumber leaves. Note the lack of dark, spongy masses on the back of the leaf.
Pseudomonas syringae pv. lachrymans is
seed-borne, and infestation occurs beneath the seed coat, so the cotyledons are
infected upon germination. The bacterium grows and colonizes leaf surfaces in
the intercellular spaces of leaves. Raindrops, insects, human field workers and
farm equipment can spread the inoculum from leaf to leaf and from plant to
plant. The bacterium enters stomata and tiny abrasion wounds. The bacterium can
survive for up to 2.5 years in dry leaves and overwinters in crop debris that
has been infected.
Utilizing a thorough set of control measures, angular leaf
spot can be reduced. Using seed free of pathogens is the first step in primary
control. The number of bacteria in the cucurbit seed can also be reduced but not
eliminated after 20 minutes of treatment with water at 50oC
containing different acidic organic compounds. Repeated applications of copper
sprays as a foliar protection can be beneficial in warm, humid locations where
the bacterium is endemic.
Summary:
It is essential to determine which of these diseases you are
dealing with as, with one being a fungus-like oomycete and the other a
bacterium, the control options available (such as fungicides) differ
drastically, and one control measure may have no effect on the other disease.
By looking at the key differences in these two pathogens, as described above,
one can differentiate between these diseases that cause similar symptoms on
cucurbits. In 2023 the Perennia Plant
Health Lab opened its doors to accepting disease plant samples in addition to
its virus testing services. At this time the focus is on meeting the needs of
the Nova Scotia agriculture industry, however if you are interested in learning
more about what diagnostic services provided, please follow https://www.perennia.ca/agriculture/plant-health-lab/.
Our team of plant disease diagnosticians and specialists, with the aid of
conventional and molecular diagnostic tools, can accurately identify what
disease you are dealing with and provide tailored recommendations for your
unique growing operation.
