Alternaria alternata (Fr.) Keissl.

Alternaria alternata is a fungus causing leaf spots, rots, and blights on many plant parts, and other diseases. It is an opportunistic pathogen on over 380 host species of plant.

It can also cause upper respiratory tract infections and asthma in humans with compromised immunity.

Hosts and symptoms

Alternaria alternata has many different hosts depending on its forma specialis. Alternaria alternata f.sp. cucurbitae infects cucurbits. Alternaria alternata f.sp. fragariae infects strawberries.

Alternaria alternata f.sp. lycopersici (AAL) infects only certain cultivars of tomato plants and is often referred to as Alternaria stem canker of tomato. AAL's main symptom is cankers in the stem. It resides in seeds and seedlings, and is often spread by spores as they become airborne and land on plants. It can also spread throughout other plants. Under severe infection, lesions enlarge and become coalesced causing blighting of the leaves. This symptom progression occurred in research done in Pakistan: the symptoms on affected tomatoes started with yellowing and browning of the lower leaves, then began developing on the leaf tips and along the margins of the leaf petiole. This progression continued until the entire leaves were covered in diseased tissue and then fell off. In addition to necrotic leaves and petioles, plants are found to have severe defoliation, with considerable yield losses when it occurs before flowering. The tomato fruit can also be infected as well, with brown cankers dotting them and making them inedible. Once the disease has spread to a certain point, little can be done to save the tomato plant.

There are several host factors that affect disease development. For example, various signaling pathways in tomato plants affect their susceptibility to AAL. Salicylic acid promotes resistance to AAL and antagonizes the ethylene response. Ethylene controls the synthesis of jasmonic acid, which is a necessary pathway for immunity. Independently of each other, salicylic acid, ethylene, and jasmonic acid can influence the susceptibility of tomato to AAL. Diagnosis of AAL is often from observing signs and symptoms from this fungal pathogen. In addition, a tomato cultivar's resistance to a toxin produced by AAL also affects disease development.

Environment

In order to survive, Alternaria alternata needs a moist warm environment. It is often found in areas with humid climates, or where there has been significant rainfall. The fungus lives in seeds and seedlings, and is also spread by spores. This disease flourishes in dead plants that have been left in gardens over winter. Additionally, when dead infected debris is added to compost pile it can spread to other vegetables throughout the garden.

There are no insect vectors for this disease. This means that using insecticides has no effect on the spread of this pathogen. However, there are several cultural practices that can be followed to suppress this fungal pathogen's impact. The disease first occurs in the host's exposed leaves. Plants planted with rows in an east–west direction have more severe disease than do plants planted north–south. This implies that if one plants tomato plants in a north–south manner they will be less susceptible. It is also suggested to highly monitor plants in April through June. This is when the pathogen is most prevalent. If monitoring indicates the presence of AAL, it is suggested to begin late-spring treatments of fungicide about mid-April. However, if a garden has a history of disease, it is advised to take extra measures. This can be done by treating tomatoes in mid to late April and 2 to 3 weeks later by applying a fungicide. Because this is a fungal pathogen that thrives off of wet environments, overhead irrigation is never advised when irrigating. This causes the moisture to remain on the leaf tissue and increase susceptibility to the disease and provides an optimal environment for the fungus to survive and grow. Furrow irrigation or drip irrigation systems allow the plant to remain dry.

Maddenin devamı Vikipedi'de