Melon - Climatic and environmental accidents

Climatic and environmental accidents

- Thermal stress

Cucurbits are rather temperature sensitive. Field crops of melon and to a lesser extent under shelter, which are subject to climatic hazards, sometimes present very damaging pathologies expressed on the fruits as well as on the leaves.

Sunburn on fruits

The fruits of cucurbits are very delicate organs. Their soft tissues and waterlogged be "dehydrated" superficially when exposed to sunlight (sunburn ) ( sunscald , sun burn ), primarily on its most exposed side. Thus, large irregular patches, whitish to grayish, appear (figure 1). They gradually cork on the surface and cracks and bursts can form as the fruit grows (figure 2).

Almost all the fruits of Cucurbitaceae can be affected, in particular when the foliage which protects them has been more or less destroyed for various reasons: aerial pest attack, manifestation of the foliage grid ... Note that these superficial lesions can be invaded by various fungi that secondary saprophytic cause rots.

Physiological yellowing of fruits

Producers sometimes observe a physiological yellowing localized on the still green ribs of smooth Charentais melon. Thus, large bright yellow areas depreciate the external appearance of the fruits (Figure 3).
This yellowing is the result of a local reaction of the bark exposed to the sun; it tends to fade as the fruit ripens. It should be noted that certain varieties are more sensitive to it, in particular the varieties with rather limited foliage, often of the monoecious sexual type.

Leaf sunburn

The effects of solar radiation can also be manifested by strong condensation on the leaves , especially under small caterpillars or under other shelters. The many drops of water present on the limbus then play the role of a magnifying glass in the presence of the sun. The underlying leaf tissue is scorched and rather well demarcated and necrotic spots appear on the leaf blade (Figure 4).
This climatic accident has no consequences for the plants. In such a situation, it may be advisable to ventilate the shelters because the humid climate could favor the development of aerial pests.

Corky stylar scar on fruit

This non-parasitic disease , which is not frequent, manifests itself on the fruits by the development of an excessively large and corky stylar scar (figure 5). It is particularly prevalent on early crops (under cover and open fields) which encounter unfavorable climatic conditions, in particular temperatures that are too low during flowering and fruit set.
This disease hardly affects the varieties of Charentais melon of the monoecious type, these varieties in general have a rather little marked stylar scar. On andromonoecious varieties where the scar is usually larger, it can be very detrimental.

Crown and corky roots not parasitic

The roots and crown of the melon have a natural tendency to suberise easily when they encounter unfavorable environmental conditions in the soil, such as too low temperatures, for example, with or without excess water. The collar with the corky cortex also shows more or less significant longitudinal bursts (figure 6).

- Hail

Thunderstorms accompanied by heavy rains and hail can cause lesions on all the aerial organs of the melon. In fact, hailstones, by striking the leaves, cause holes and tears which can greatly alter their photosynthetic potential subsequently, some leaves being completely shredded (figure 7).

Hailstones are also the cause of brownish necrotic changes, more or less pronounced bursts at their point of impact on the stem , petioles and peduncles (figure 8).

Young fruits melon are very fragile to hailstones which can cause them indelible lesions which will subsequently make them unmarketable. Corky spots, bursts corresponding to the points of impact of the hailstones will dot the fruits (figure 9).

In general, the distribution in the crop of affected fruits will be generalized.

- wind Strong

The young wrinkled melon leaves, chewed by a strong wind, subsequently show necrotic inter-vein lesions (Figure 10). There will often be a lag between the effects of the wind and the expression of leaf symptoms which will be later on more developed leaf strata.

- Various injuries

As we have already pointed out, the young melon fruits are very sensitive to external aggressions. The slightest mechanical injury can have considerable repercussions as the melons grow. For example, the latter could be deformed by corky cracks (FIG. 11) making them difficult to market later.

- Pollen deposits

Growers are sometimes surprised to observe a few tawny-colored spots on the surface of melon leaves and fruits. These are slightly raised and have a powdery appearance. When you rub them with your finger, you can see that they are in fact superficial deposits that can be removed very easily. Their origin can be easily elucidated: for this it suffices to observe the bees in the melon crops that they pollinate. Their legs are loaded with numerous aggregated pollen grains in the form of sometimes bulky "balls". The latter, when they have reached a certain size and during periods of wet weather, fall on the leaves (figure 12) and fruits (figure 13), particularly of melon or zucchini.

These deposits are without consequences for the affected organs; they can depreciate their commercial quality.

Last change : 05/03/21