During wet periods the risk of infection by leaf spot fungi is bigger.
Leaf spots caused by fungi appear in most cases during periods with a lot of rain, high temperatures and a less active plant.
The most risky factor is not enough ventilation and a crop which does not dry up fast enough. A high temperature and high relative humidity are also necessary. The outside flowers face this problem directly but also in greenhouses without heating or when heating is switched off the risk are high. The lack of ventilation causes a less or inactive plant and long periods of leaf wetness. There should be air circulation between the plants. A lower plant density will increase air circulation. In greenhouses ventilation should be more aggressive, faster opening. Open both sides or open the side windows of the greenhouse.
Internal plant quality
The fungus can be relatively stronger in case the plant is weaker. A high Relative humidity (RH) causes softer and weaker plant cells and stomata which are always open. Because of the high RH evaporation is difficult and gutation appears. In case cells burst the cell fluid will be feeding substances for fungi. In case of outside flowers a too vigorously growth should be avoided by fertilizing at a lower level. A lower nitrogen gift will reduce leaf size and very soft leaves.
Weak cells can also occur in case the feeding is too low or in case of calcium deficiency.
In crops like roses and gerbera the total amount of leaves can be reduced by pruning more. In outside flowers the weeds should be removed so these plants are not a source of diseases.
A wound is an opening for many fungi to start infection. The plants also use extra energy for restoring the wounded area.
Damage can occur because of the nets or wires between the plants, the nets which keep the crops upright. In case the netting is pulled higher and in case of other work between the plants this has to be done with great care. Especially in case the plants are wet, so more heavy and more on pressure, the change of breaking is bigger. Also heavy rain, winds and sandstorms can damage the plants.
Leaf spot fungi
The spreading of typical leaf spot fungi is often through the air, examples are mildew and rust. Other fungi will spread through splashing of water in the crop. The weather influences, sporulation and infection. Sporulation is the moment the spores of the fungi are released into the air, like pollen from a flower. The spores land on the leaves, germinate and will try to start infection. For germination most fungi need water, so a wet leaf. Mildew can germinate without free water. The period that the leave has to be wet and infection is successful differs for every fungi. Fast drying of the plants is for this reason important.
Well known leaf spot fungi
Primary: Mildew, Rust, Septoria, Fusarium, Colletotrichum
Secondary: Phoma, Cylindrocladium, Alternaria, Botrytis
A fungi which is normally secondary can become primary in case the plant is weak.