1. Introduction
Hemp (
Cannabis sativa L., Cannabaceae) is considered one of the oldest crops known to man [
1]. It is estimated that its use dates back to 10,000 years ago [
2] and a hypothesis of co-evolution of the genus
Cannabis with the human species has been postulated [
3]. The hemp plant is native to India and Persia, although it has been cultivated in nearly all temperate and tropical countries [
4,
5]. Hemp is a versatile herbaceous crop that has been used for fiber, food, and medicinal purposes [
6,
7,
8,
9]. Hemp is an inexpensive and available bast natural fiber [
10]. Fiber production from hemp has been conducted over many centuries, for end uses ranging from textiles and papers (hemp paper was used in the first copies of the Bible) to ropes and sails [
11].
The cultivation of hemp dates back to China around 2700 BC and is believed to have then expanded across Asia, making its way to Europe 2000–2200 years ago [
3,
4]. As a multi-use crop, hemp is considered one of the oldest plants cultivated to provide nutritional and medicinal benefits [
7,
12]. Also, hemp provides raw material to a large number of traditional and innovative industrial applications [
1]. In recent years, the interest in investigating the potential use of industrial hemp in food and nutraceuticals has been growing [
13]. Pressure from weeds, insects and diseases are among the major challenges in production of
C. sativa [
14]. Fusarium species can parasitize a wide range of plants, including vegetables, flowers, field crops (cotton, hemp, and tobacco), herbaceous perennial ornamentals, and plantation crops (banana, plantain, coffee, sugarcane) [
15]. Also, they can parasite vertebrates, insects, humans, or even other fungi. Conducted mycological analysis of seed and grain of many crops from 2002 to 2008 in Croatia revealed the presence of
Fusarium species on wheat, barley, oat, triticale, maize, tobacco, bean, pea, soybean, lupin, vetch, alfalfa, clover, flax, beet, spinach and lettuce seed and grain [
16].
F. oxysporum has been confirmed as a cause of wilting soybeans in Croatia [
17].
All parts of the plant at all growth stages are infected by one or more pathogens while pathogens in the genus
Fusarium are among the most destructive pathogens of
C. sativa [
18]. Several
Fusarium species attack hemp crops and cause damping off, including
Fusarium solani, Fusarium oxysporum [
19] and less frequently
Fusarium sulphureum, Fusarium avenaceum, Fusarium graminearum, Fusarium culmorum, Fusarium avenaceum and Fusarium fujikuroi [
18]. The time of infection and vegetative growth phases results in decreased plant quality or even total plant loss. Species of
Fusarium also cause significant postharvest losses and produce mycotoxins that further limit the value of the crop [
20]. Because, for many countries, the legal status of the plant changed from crop to illegal drug (1950s) and back again in the 2000s [
21], the bulk of literature on
C. sativa and
Fusarium is written in two distinct time periods (prior to 1960 and after 1990). There are some exceptions to this bimodal distribution, particularly publications reporting the use of
F. oxysporum f. sp.
cannabis as a biological control of the plants [
22].
F. oxysporum is a common fungal pathogen that causes wilt disease in nurseries and in field cultivation and causes high losses [
23]. Some of these fungi also cause cotyledon drop [
24]. In mature plants these pathogens cause Fusarium root rot, stem canker, and wilt [
25]. Fusarium wilt is the name of a disease caused by two microscopic fungi that infect hemp roots and then move systemically through the plant. Symptoms of Fusarium wilt begin as small, dark, irregular spots on lower leaves [
26] and pathogen-affected leaves suddenly become chlorotic (yellow). Wilt symptoms begin with an upward curling of leaf tips. Wilted leaves dry to a yellow-tan color and hang on the plant without falling off (
Figure 1). Petioles, branches, and sometimes even the stalk may droop while stems also turn yellow-tan. Cutting into wilted stems reveals a reddish-brown discoloration of xylem tissue. Pulled-up roots show no external symptoms. Fusarium wilt is a warm-weather disease, and optimal temperature for fungal growth is 26°C. Disease symptoms may not become evident until the advent of hot summer temperature [
27]. Fusarium conidia spread via splashed rain and water runoff.
Fusarium spp. can live saprophytically on roots, stems, leaves, flowers, and seed of diseased and dead plants [
28]. The fungus can survive on seed (internal and external) or as spores or mycelium in dead or infected tissues. The pathogen is disseminated by air, equipment, and water [
15].
Fusarium wilts are of worldwide importance and disease severity is favored by warm climates and warm soil temperatures [
18]. Wilt development is favored by air temperatures of 28°C, low levels of nitrogen and phosphorus, elevated levels of potassium, low soil pH, short day length, and low light intensity [
15]. In other hosts, virulence of
Fusarium can be enhanced with ammonium nitrogen, and decreased by nitrate nitrogen [
29]. Many species of
Fusarium co-exist with their hosts as endophytes and protect the host against pathogens and
F. oxysporum has been reported as endophytic in
C. sativa [
20].
F. oxysporum is typically soil borne and is the most economically important and the most commonly isolated species of
Fusarium [
30]. Due to the large numbers of spores produced, oil infested with
Fusarium may remain so indefinitely [
31]. Seed-borne infections lay dormant until seedlings sprout the following spring. Mycoherbicide researchers reported that spore-coated hemp seeds effectively spread
F. oxysporum through the soil [
32].
Fusarium wilt reduces fiber quality and fiber yield and reduces seed production and seed quality. Seeds infested by the wilt fungus should not be used for human consumption, or for cultivation or breeding purposes. Mycotoxins, produced by many species of
Fusarium, are of greatest agronomic importance limits the food and feed supply [
18]. Mycotoxins can create serious health problems in humans and animals [
33].
3. Discussion
In this study, according to cultural and conidial morphology, ITS4/ITS5 and EF1/EF2 sequence based phylogenetic analysis and pathogenicity test, the pathogenic fungus was identified as
F.
oxysporum, and confirmed to be the causal agent of Fusarium wilt in hemp (
C. sativa). To the best of our knowledge, this is the first report of hemp Fusarium wilt causing
F.
oxysporum in Croatia. Considering that
F.
oxysporum has been reported not only in field crops [
16,
17] but also on many weed species [
35], the presence of this pathogen could cause economically significant hemp production decreases.
F. oxysporum attacks hemp seeds causing a decrease in quality. Fibranova [
36] hemp cultivar was used in the pathogenicity test because our collected isolates were found in the field sown with this cultivar. The authors did not conduct a statistical analysis during the trial because the aim of this study was nothing but to simply determine the existence of a new pathogen which causes disease on hemp plants in commercial field in the Republic of Croatia. Although is to be assumed that there may be differences in the susceptibility of cannabis varieties to this pathogen, the susceptibility of this variety was not the subject of this study.
In the study of Ivic et al [
16] the authors list the crops in Croatia on which they found the presence of
Fusarium species, but do not determine them to Form species (
formae specialis) depending on what hosts they are to infect.
Using the described methodology, morphological characteristics and based on molecular analyzes, we determined new isolate HFox1. We confirmed pathogenicity of HFox 1 isolate to hemp by applying Koch's postulates. The sequences [
21,
37] of isolate (GenBank accession no. OM475708 and OR149071) was compared with those available in the NCBI (National Center for Biotechnology Information,
http://www.ncbi.nlm.gov/; accessed on 22 March 2022 and 12 July 2023) genetic database by using the Basic Local Alignment Tool (BLAST) algorithm showed 100% similarity with
e-value of 0 to the reference sequences of
Fusarium oxysporum (GU724513 and KX196809), as well as
F.
oxysporum (MK461973).
Hemp was regularly grown in eastern Croatia after, and especially between the two Great Wars. By the middle of the twentieth century, the largest producers will remain concentrated in parts of Russia, Ukraine, Hungary and former Yugoslavia [
38]. During the former Yugoslavia, hemp was one of the leading cultures, and the country itself was 3rd in the World in hemp production [
39]. The area of Slavonia was significantly oriented towards cultivation, processing and export of industrial hemp [
40]. In the 1960s last century, hemp production was almost completely abandoned by passing laws that prescribed strict conditions for industrial cultivation hemp [
40]. With cessation of production as was the accompanying processing industry.
The industry hemp in Croatia has had a sharp increase in the past 5 years [
39]. Production has been in constant increase, from 1560 ha in 2015 to 2476 ha in 2019 [
41]. Upon the resumption of cannabis production in recent years in Croatia, the diseases have not been identified, or they have not been a problem nor have they been reported by producers. With the liberalization of the legislation, hemp production is expected to grow in the coming years. Narrowing the crop rotation could potentially increase the problem with diseases in general as well as the disease caused by
Fusarium, especially in humid and warm springs and more susceptible varieties [
15,
27].